I can only speak for myself, so here's some input:
- plan ahead, where you go, what will happen, ammo, weapon usage. Intuition + experience will help you. This time I was definitely not in the mood of repeating >500 tics sequences in case of mistakes, so I just accepted them and tried to readjust the strategy. Besides, a 1:00 run will be more accurate than a 10:00 run unless I want to spend my whole life on the latter...
- give yourself chekpoints. Switches/monster dead/item pickups are obvious ones. Say you open a door on tic# 2454, you may leave it like this, or try to rebuild the last 100-500 tics to check if you can manage to do it earlier, on tic #2448, or even earlier, etc. until you'll satisfied with the outcome.
- No I don't rewatch the demo every 2 tics. When it's just running, I constantly check player's speed. If it drops, probably I hit a wall, this should be avoided. If it increases from about 23 to above 30, perfect, I managed a wallrun. Normally if it increases until maximum and stays like this, and you're moving in the right direction, you're doing fine. When it's fighting and you need these monsters dead NOW, you repeat until you're satisfied (you know how many rockets/SSG shots you need). I have a Doom Builder open to check for monster ID and a PrBoom+ open to check for damage (trace_givendamage [playerID number] / trace_thingshealth [3 chosen monster ID numbers]). I quickly save, choose 200 different filenames so I don't overwrite something which is just perfect, copy to PrBoom+ folder and watch with -skipsec. If I plan on a 2 rocket hell knight kill AND I only deliver the first rocket at the moment, then do something else and return after a while, I make sure this first rocket is at least 250 hp damage. If the hell knight is to be dead now, the first rocket damage helps, but may be lower, like 230, I check if the second one is enough (in other words, I repeat only about 40 tics in the latter case).
- I find waiting tics stylish. You may shake and turn around like crazy, but it doesn't help to create a watchable demo. Whether you'r on an elevator or in between SSG shots, you can't make these things go faster, so you wait. WT ticks are also considerably faster to create, as they need just to be cloned. When the monster is about to die, you start moving and/or setting the next target. Watch the last seconds of dark2-06 (ssging a baron). The player positions himself right next to the exit switch after the 4th SSG shot, so no time is wasted. This is an example of understanding TAS combat.
- The whole thing is a bit (too) time consuming, like making a sculpture perhaps, however watching what you've just done, even 15 seconds portion of the demo, can be very rewarding