My favorite general trick is the cursor trick. For almost any precise jump in the game, place your cursor in a certain spot (such as where you intend to double jump) to help you with your positioning. Obviously you should determine the best locations for yourself on a lower difficulty. This trick is really useful in places like the precise egg rocket jump after Mecha Birdo, the spike jump where the paths after the KKK and Mecha Birdo meet, the part in the Final Tower of the Guy where you have to fit between those 2 spikes (place the cursor beforehand), and surprisingly Link.
Never take the lower path. The second time you need to enter the Factory, just enter Kraidgief’s warp in the Room of Divine Transportation
Timing is a major in 3 areas: Castlevania, Guy Industries (after Mother Brain), and in the Mansion of the Guy. For Castlevania, the pattern of the Medusas is determined by how long it has been since you entered Guy Industries (right after Mecha Birdo). So the moment you enter the area, look at a clock that records seconds to find out the exact time, and go through the area. When you are about to jump into the fan, wait until, say, exactly 3 minutes have passed since entering the area, and learn that Medusa pattern. This way, you can just learn a single pattern and not worry about randomness.
For Guy Industries, there is the first block of the Snake Room. It can only disappear at specific times, which is why when you jump onto it, it takes an inconsistent time to disappear. Even worse, it disappears so quickly that if you wait for it to look like it is disappearing, you will instantly fall through and die. IIRC, if you’ve been in Guy Industries for exactly 2 minutes, it will disappear almost instantly. So wait until then, jump on it, and immediately jump towards where the next block appears.
In the Mansion of the Guy, the patterns of the apples in the Bowser room and in particular the Lu Bu room sort of become randomized over time. Instead of using a clock, learn how to do each part in the fast way possible (which usually means jumping onto a platform at the earliest opportunity). That way you will have a consistent pattern.
After KKK, it is possible to fit between 2 spikes of the falling ceiling. It is pixel precise, but there is a way to do it reliably. First, jump into the right wall of the KKK boss room after you win, and don’t touch any other walls (NO QUESTIONS ASKED). Now, you want to fit in between the second and third spikes. Use the cursor trick. Practice this a lot beforehand, learning exactly where to put the cursor (use the background as a guide), and how the Kid should look in relation to cursor (i.e., how much of his head should be to the left or right of your cursor). After the ceiling goes up enough, quickly finish the room. You obviously won’t have to worry about the dodging the trigger anymore.
You probably already realize this, but for Mother Brain, leave nothing up to chance. Have a strategy for her completely memorized, knowing each Spaghettio and shot to dodge and when.
Figure out which path you need to do in your Impossible run first. It might be the right path because of G&G (there’s nothing wrong with skipping Mike Tyson and going back later) or the lower path if you have trouble with that spike fall leading to Mother Brain (this is the exception to the “no lower path” rule).
To prevent the game from crashing, follow the advice of the post that the FAQ links to.
Here are some tips for the mine cart ride. At the beginning of the second section, instead of staying on the cart and trying to kill cheep-cheeps, jump forward and run forward constantly shooting. Whenever the cart catches up, jump forward again.
At the beginning of the third section, in that first floating metal/jump pad thing, instead of using the detached part of the pad and risking going downwards into that spike, it is better to walk off onto the attached part of the yellow pad, where there is no risk. It took me a while to figure this out, even though it seems so obvious now.
Finally, here's one for that last part, with those several jump pads in a row. When you reach the first jump pad, wait a few seconds for the cart to move forward. Don't walk all the way into the jump pad; optimally you're just touching it with your face. For some reason, you go further by doing this. During this section, hold down the shift button to preserve your double jump. Finally, after the last jump, you'lol either go upwards or down. Obviously, if you go up, just use the floating pad, but if you go down, use your double jump to reach the land.
The last piece of advice I can give is that for any part you have trouble with, practice it a lot, and try to find alternate strategies or actually try to create a strategy in the first place. I’ve created tons of very specific strategies for pretty much every part that I obviously can’t share with you (unless maybe I tried to create a video walkthrough, which I won’t). Relying too much on instinct can be a bad idea. Of course, the same goes for intelligence, especially in all those random boss fights.