Here are some tricks, some tips, some advice, and some total bullshit. You decide.
1. Cutting back plants.
This is done to prevent the (perennial) plant from blossoming too soon, or getting too tall, or both.
Example: Perennial aster. Just regular, run of the mill, big vase shaped perennial aster. If you cut this off by one third it's height in May, it will stay compact, the plant form will broaden, and blossom time will be moved back from July to mid-August. Believe me, in August, you want a big, massy fan of color, because your lawn is dead, everything is wilting and setting seed and looking ratty. There in the middle of your post Apocalyptic garden is *cue angel song* Your Aster! Glowing! Thick with bloom! Native pollinators are loving you! People smile as they drive by!
The same can be said of most herbacious perennials. The longer you keep them from blossoming, the broader they will become, and the more abundantly they will produce blossoms.
NOTE: Only do this once to any herbacious perennial. You might get away with it twice with a centranthus ruber, or a monarda, or some of the coarser campanulas, but more than three times is killing it. All you guys reading this are laughing now. I know you are.
2. Cutting back shrubs/woody perennials.
Anything that has bark is a woody perennial, and this is basically shrubs and small trees.
OK. With this the rule of thumb is, cut hard to shape in February or March, and then cut to thin after blooming (if it blooms.)
Here is why you have to thin, which means cutting off mass.
Small trees like ninebark and magnolia will start putting out the branches right after they bloom. And that is fine, until over time the plant gets as thick as you want it to get. After that, go up in there after it blossoms and start cutting out small branches (mass,) or else you'll end up with a huge beast come winter, and if it ices over, that small tree might very well end up torn apart by the sheer amount of ice that can form and add dead weight to all that mass. I lost a gorgeous Mountain Ash like this - and Mt. Ash qualifies as a small tree, even though they get huge here. We got an ice storm, I hadn't bothered to cut out the mass, and come winter it iced up and half of it tore away from the trunk. The whole tree died two years later - and that was a tree that I'd been carrying around with me from rental to rental for fifteen years before I planted it here. It was fucking heartbreaking.
Star magnolia is a waterhog - meaning the more it's watered, the more it grows, and that watery growth is weak and excessive. Keep an eye on it, and if it's looking really massy - thick and impentrable - in the canopy around July? Get up in there with the loppers. I have a mature specimen that I have to take care of in midsummer and early winter, and it's absolutely worth it because STAR MAGNOLIA.
Buddleia is also a waterhog. If it gets overwatered during the summer, which is something that I am guilty of, it will put on all this huge angular weak, heavy mass. Come winter it will literally tear itself in half in a high wind, or given a very rainy week. Fortunately, assuming this has happened and you're standing there going "Oh shit my buddleia tore itself in half" you can choose the half you like, leave it rooted there, and cut off the other half; just completely cut it away at the point of breakage. Boom. Then cut away half the height of the part you are keeping, backfill with compost if necessary, and haul all the discard out to the compost heap. It WILL come back.
Hazel/Filbert: It's a funny tree. You can let it grow as it will from seed, and in four years you will have a very nice tree.
That is also when it will begin suckering. And I mean suckering like a mad motherfucker. Sudden straight poles that do nothing but have leaves without fruit will come shooting up from the trunk and the branch axils for no good reason, the way an apple tree does. You always have to be cutting on a hazel, and you have to be very careful about sanitation. Seal the cuts with Vasoline.
3. Sanitation in the garden.
No I am not high. Make sure your loppers, secateurs or sawblade has been swizzled in a bucket of bleachy water. About one teaspoon of bleach to your basic plastic pickle bucket of water. Sozzle that blade around in there between each cut, if you are working on a single plant that you suspect has a fungus or other disease. When you move to another plant, sanitize your equipment again. Slosh it around in the bleach water.
And speaking of excessive sanitation and bullshit, the Cornus Californica...my favorite dogwood tree in the world. It is a water hog, it will come on with lots of soft, heavy mass, and even with benign neglect it wants trimming almost constantly because the branches take off in weird directions for no logical reason. This happens in the wild, too.
Unfortunately it is also horribly prone to Anthracnose, which is a fungal disease that roses are also prone to. This shit took out my beautiful, beautiful tree after fifteen years, and I blame myself for having gone from trimming off an infected rose bush right to the dogwood, without sanitizing my tools between jobs.
I kept it going with my sulphur-water-castille mix for another two years, and then I just cut it off slick with the ground.
Once you get a badly diseased plant in the garden, tear it out AND BURN IT. If you can't burn it, wet it all down with pure bleach. The dead wood, root mass and the dirt that clings to it will spread the disease otherwise.
4. DIVIDING PLANTS
Gasp! Clutch the pearls! This is so freaky and terrifying! Whatever shall I do?
Well I'll tell ya.
Do not divide plants that are in bloom. Period.
If you must, cut off ALL THE BLOSSOMS FIRST. This way the plant will receive a biochemical signal telling it to direct its efforts to building callus on severed roots and growing new roots to reach for more sustenance.
If a perennial plant has two stems that come up separately, not from a single trunk, now, but separately - from a single root mass? Stick a shovel between the stems and lever half of that plant out of the ground. Backfill the hole with dirt or compost, water it until it puddles, and walk away with your brand new half a plant. You can stick it in the ground somewhere else, or you can compost it. Your choice. It's just that simple.
If there is a huge clump of stems coming from a single root mass, like a hosta or a daylily? Stick a shovel into the middle of it and lever half of it out. See above. Just that easy.
The key is to have everything ready beforehand.
First, cut off all the flowers and seedheads. Have ready at hand a shovel, hose, (hole dug and watered in heavily to receive the division) sharp shovel, keyhole saw to get through the roots if necessary, and dirt or compost to backfill. All that in place and ready, it will go smooth as silk. No fear. No secret substances or unnecessary surgical procedures. Water the half a plant that's staying, backfill, water in again, and walk away.
Expect that these halves will look like shit for a week or more. (Particularly the one you threw into the compost heap, right.) Then you will begin to see signs of new growth. If there is a hot, windy spell of weather then water the divisions again, deeply. Once.
This is pretty basic shit, kids. Plants are pretty basic things. (I am not counting raising orchids or conservatory growing here because I know exactly jack shit about those methods.) True, every plant does have conditions that it prefers. Go to Daves Garden.com for more articles, information, answers to your questions, and growing information on just about every plant and variety of plant you can imagine.
Here is a screenshot of the Plants and Information page.