At a 20 handicap, a little bit of game planning can lower your handicap a bit. How well do you know your home course? Where are the scoring opportunities?
For the par-3s, where should you be aiming on each green? Which side should you be avoiding? Which club(s) do you hit on those tees, and do you hit them in warm up, picturing the green? Where are the best places to miss?
How do you play the par-5s? What does it take to hit a short iron you're comfortable with (say, a 9- or 8-iron) into each for your third shot? That is, what is the easiest route to hitting the green in regulation?
At my home course, there's a par-5 where I can hit a 3-hybrid off the tee, a 6-iron down the fairway, and then an 8-iron into the green. There aren't any significant ridges or other impediments to finding the right line to the pin. If I'm smart when I play that hole, I can hit it in regulation (unfortunately for me, I don't get a shot on that hole, as it's the #17 handicap; such is life) and should have a good shot at a short putt for par. Unfortunately, because of the hole's design, a great drive also lets me hit a fairway wood to where I can pitch or chip for my third shot, and I get greedy (one of the hazards of gaining distance, without being a long driver, when you're used to all par-5s being three shot holes: you start to want to try for getting home in two, whereas consistently short hitters don't try for this and consistently long hitters can do this easier)
If you can average a bogey on each par-3, and play your par-5s in +2 total (assuming four of each), you can average a little over bogey on your par-4s and break 90.
For the par-4s, which ones are easy and which ones destroy your score? Are there any that are easy bogeys and tough pars? Maybe take the easy bogey on those. How about the short ones? Back up a short iron you're comfortable with (again, say 9- or 8) and determine the easiest path to there.
One thing I've debated doing, maybe others can chime in: take your last X scorecards from your home course and plot each hole, relative to par, in a spreadsheet. Determine which ones you're playing the worst (again, relative to par). Suppose you're averaging near +2 on a par-4. That may be one to treat as a mental par-5, and see if there's some easier way to play it. Is it long and you're stretching a driver and fairway wood to get there, or is it short and you're trying to do too much off the tee?
Now, one hazard of this: don't get too used to laying up to a full club, because that isn't always the right answer. In many cases, being 50 yards off the green lets you get on the green - and closer to the pin - than being 100 yards away.
Speaking of which: what are your quarter (shaft back to parallel) and half (arm back to parallel) wedge distances? If you're 45 yards away from a green with no hazards in front, what are you hitting and how?