I've gradually discovered a few time-saving tips:
1. I try to only work in the top quadrants, since that's all you need to submit to complete a puzzle. So no filling in Xs next to Dots unless it's in the top quadrant.
2. I skim quickly for clues that directly allow a Dot in the top quadrant.
3. If the puzzle has a low solve rate, I start at the bottom with the clues.
4. I save "the 5 intrepid teenagers in the Mystery Machine were..." clues for last, as they can often be done very quickly once you know how a couple of them map to the top quadrant.
5. Never, ever, come back to an unsolved puzzle unless a 100% solve rate means something to you. I managed to screw up my average solving time for May with an 11,000-second-plus time. The clock runs even if you don't.
6. Don't be afraid to try any puzzle. Even if you see something with a 20-minute average solving time (1,200 seconds) and a 10% solve rate, there's often a way to attack it. For those with a really low solve rate, I will violate rule 1 above.