From my little bit of tourney experience I'd say this:
1: Brawl is a more defensive game: this means you opponent will most likely employ "wall o pain" strats projectile spam, and other forms or camp to be as safe as possible. why not? its a tourney, there is probably money involved. be care not to get caught easily in those traps.
if you see it, being able to change your strategy against a character is what separates the better players.
2: You have 7 minutes. You don't have to nessesarily rush to kill them three times, and risk losing that one stock advantage you had. Most often once they get the first stock on you, they play the time factor more to force you to make more mistakes and dig yourself a deeper hole. yes you are fast, but knowing when to go full speed, or when to slow it down within those 7 minutes will be key for you. And if you are ahead a stock, play the percentage game, and make them work to get the kill off you, so that when they finally kill you, you are still ahead percentage wise. atm, if the timer runs out, and both players have tied stock, then the one with the lower percentage wins.
3: The average person who plays even a decent sonic, will camp. Sonic is just too fast to chase around all day, even if they have experience agsinst him. Theyre going to attempt to wait for you to come at them, and time a counter at some point in the match, and sometimes, righ toff the bat. you have to expect it, and change it up the second it looks like that is the case, otherwise you just gave them the pace of the match.
4: I for one before most Brawl sessions with other people, will warm up with simple running and movement oriented excersizes. Im not kidding. It's your number one asset, and a lot of the match will eb determined by how well you can move around when you fight.
5: If there are any Sonic mains/Players there, play them. ask questions, and you may find somethings interesting to try for the next time you enter.
6: Lastly: you better have a kick *** secondary to fight with to help balance out sonic's weaknesses. he doesn't have a projectile, has to attack close, etc. you know it already. so pick a good secondary now and master him too. For me right now, I'm trying to master Meta and Rob but for the moment my secondary is Dedede.