Yeah, I know "know your deck" seems obvious, but honestly, find something that makes you feel good to play and then keep playing and refining that for a while and see where it takes you.
Most recent tournament I went to, I didn't do so well, but got a lot of experience, had a lot of fun, and had 20+ minutes of relaxation every hour because the games were done so quickly. NEH/Whizzard.
Mulligan testing can be really good, as mentioned above, even just talking about it with your friend if you decide to mull, see if that was the right move. Bad to give out info in a tourney, but for just having fun that can matter a lot. Also helps the decision stick in your head a bit more after, so you can see if it was as solid a choice as you thought at the time -- did you need that Plascrete after all? If you like the idea of new decks but don't want to risk playing them all, put them together on the DB and try a few sample hands.
Multi-access, trashing, and otherwise shutting the corp down all definitely feel like they lead to shorter games, to me. One reason I love Anarch so much, no influence spent for Imp, Medium, Nerve Agent, or Parasite.