There are plenty of methods used to unlock outfits. Here is a list of them:

• Shutout the CPU in at least 1 Game in a Match
• Won a Match using a Backhand Shot
• Won a Match using a Forehand Shot
• Won a Match using a Smash Shot
• Won a Match using a Slow Shot
• Won a Match using a Lunge Shot
• Won a Match using a Heavy Backspin Shot
• Won a Match using a Heavy Leftspin Shot
• Won a Match using a Heavy Rightspin Shot
• Won a Match using a Heavy Topspin Shot
• Won a Match using a late swing
• Won a Match with a Really Fast Shot
• Won a Match with a 4 Point Deficit Comeback
• Won a Match with a 5 Point Deficit Comeback
• Won a Match with a 6 Point Deficit Comeback
• Won a Match with a 7 Point Deficit Comeback
• Won a Match with a 8 Point Deficit Comeback
• Had a Rally of at least 40 Hits
• Had a Rally of at least 100 Hits
• Won a Match by forcing the CPU to hit the net
• Won a Match by returning a shot from the CPU that dropped short
• Won a Match by making the ball bounce 2+ times on the CPU’s side of the table
• Beat a circuit in less than 20 minutes
• Overcame a 2 game deficit to win a 5 game match
• Won at least 22 Matches with a Character (22 gets you one outfit, 23 gets you another outfit, etc)
• Win a match at a character’s recreational venue (Whatever level you unlock with a character, that is their “venue” -- example: Win a Match at the Factory using Juergen and you unlock an outfit.)

Time-conserving method:

The following achievements can be locked in this order: (explanation below them)
The Noob and The Victor Achievements
3 Match Winning Streak
4 Match Winning Streak
5 Match Winning Streak and The Widowmaker achievements.

Ok, so here's how you do the above 6 achievements for the quick 280 gamerscore points:

Get a friend, get in a private chat with them, then go to Xbox Live, Ranked Match, and have one person create a custom ranked match with unusual settings (I used 1 of 1 games per match, 7 points per game, spectators off, and 5 points per serve). Ok, when you're about to create, make sure your friend has the same settings ready to search. Tell him when you create it so he can join immediately before someone else. Then in gameplay, whoever is getting all the achievements, shut out your opponent (don't quit or you won't get the Widowmaker achievement). After game one you'll get your Newb and Victor achievements. After game three you'll get the 3 Match Winning Streak, then 4, and then after the fifth game you'll get your 5 Match Winning Streak and the Widowmaker achievements. Also, if someone does join your created ranked match before your friend does, you have the option to back out of it, so no worries there. It will take about 15 minutes or so to get these 6 achievements for 280 points.


How To Serve an Ace:

An Ace is a served ball that the opponent can’t even touch when in attempt to return it. Here’s how you do it: When serving, move about 4-6 steps to the right, and then back a step. Hit . Then look all the way left, Hit  again, and make sure your spin meter is 100% full, and the power meter can be 75%-100% full. It should serve a very nice leftspin that the computer can’t return, giving you an Ace. Please note that with each character, the amount of steps you need to take will be different. Just explore around with it and you’ll eventually get it. Also note that some computer opponents (such as Jung Soo) you cannot serve an Ace against consistently as you can against others.