I've been gathering tips to improve my time too, and here's what I've got so far:
Cross: bring it down to 2-3 seconds
- remember that you can always for the cross in 8 moves or less, learn to find that "algo", fast
- practice until you can do it blindfolded. No peeking.
OLL: bring it down to 2-4 seconds
- just practice it until it becomes muscle memory
- if you're using 2-Look, bring it down to 4-6 seconds
PLL: bring each PLL execution to sub-2
- learn how to recognize PLL cases, FAST
- find the good and easy (for you) algs in the sticky
- just practice it until it becomes muscle memory
- do PLL time attacks. A LOT.
That would bring your total time to 6-9 seconds (8-11 seconds for 2-Look OLL).
Leaving 11-14 seconds (or 9-12 seconds for 2-Look OLL) to do F2L to become sub-20.
Which is the hardest one to achieve (all the others are pretty easy with a bit of practice).
So far, the things I do to improve F2L are
- use a
metronome for practice while forming the cross and first 3 F2L pairs (you can just rush through the last one).
This trains you with 2 things... searching for the first F2L pair while doing the cross so you don't pause after the cross, and continuous look ahead during the F2L stage. Try 60 BPM first. And speed it up bit by bit.
- learn to do F2L from at least 2 angles, to lessen cube rotations and slow regrips
- keep track of empty slots, and use them to avoid cube rotations
- learn to use keyhole in some special cases. It's much easier (and faster) to use it in cases like "corner+edge inserted in the right place, wrong edge orientation", and there's still a empty slot/corner (~8 moves in contrast to the 11-12 move-ish F2L algo).
- keep your move count low, find the optimal solution to the optimal pair. Use only 7-8 moves per pair (so that you get only 27-30 moves for the F2L). Record your F2L, if you're getting 40+ moves, that's pretty bad. At 4 moves per second, that's 10 seconds (if there are no pauses and with perfect look-ahead). While at 28 moves, that's only 7 seconds.
- DON'T watch the corner and edge piece you are working on (something I read from Jason Baum). A bad habit, he says, is to connect/align a corner-edge pair, then look again on how you'd insert them. You lose A LOT of time here. Know ahead of time how you'd insert them before you connect/align them.
- F2L tricks are also nice. I just read up on a few multislotting cases, and it's nice when they come up.
- PRACTICE.