Combo stars are earned by keeping a 100% combo throughout the entirety of the course. How does one keep a 100% combo? Doing tricks, of course! Here are a couple tips to help with earning this achievement:
Know what to look for
Before selecting a stage, you can look it over to see what stars you will have to earn in that stage. If a stage does not contain a combo star, then it won't count toward the cheevo. even if you earn a 100% combo. The star looks like the achievement tile - a picture of Joe Danger's head. This type of star is often paired with other tasks, such as hitting all targets or collecting all coins. Simply put, if any of the stars in a stage contain Joe Danger's face, then it counts for this achievement.
The lab counts!
The XBLA edition of this game added a new section beyond what was available on the PS3 called "The Lab" that serves as a training grounds of sorts for the game. All combo stars collected here count toward the total.
Wheelies are your friend
When trying to keep combos, I've found often the easiest way is to just keep gently pulling back on the left thumbstick so Joe pulls a wheelie. This keeps the combo going with little worry.
Keep it simple
It's tempting to go for big moves, but to keep things easier, keep things simple till you earn the combo star. When you hit a big jump, just pull back to land a wheelie or spin once if you really feel like it. Don't try to do huge tricks as it's easy to break combo if they land wrong.
Checkpoints don't break combo
Even though time continues to run and the active notification on the lower right of the screen will switch to off when you crash, as long as your combo was active when you crossed a checkpoint, it will remain active once that checkpoint is reloaded.
Single this star out
You can replay the stages multiple times with no penalty, so when going for this, just split out the combo portion and take your time to get through it.
The game offers plenty of opportunities to get combo stars. As of this writing, I've completed the Lab and am 32% of the way through the Career, so plenty of opportunities remain, I'm sure. If you do as I did (earn every combo star in the lab and then every combo star till this popped in career), the the achievement will pop during a level titled "The boomerang'.
Good luck!