Breaker keeps tripping in my garage!

I have the same problem in my garage/workshop. Other than a new box with more capacity, I think your only cost-free option is running an extension cord from your house out to your tools or your heater. That's my solution here though you'll likely need a greater length of cord than I require.

If breaker is in garage merely connect another outlet, but at its input, not output. I presume, owever, that it's in houjse. in which case, best of course do what Blat suggests

Temptation is to replace it with one of higher capacity as Blat suggests, but that'd be a bit risky since we're not sure the cable 'tween would handle the bigger load

So Ward, howja do

your garage receptacles are on 15 amp circuit breakers 14 gauge wire, the new code this year requires all garage receptacle circuits to be on 20 amp breakers 12 gauge wire
if you have a sub panel in the garage you could have someone put a new 20 amp circuit in your garage right beside the panel. you would just get a single gang box put right next to the panel and run a short wire jump into the panel with 12-2 wire, put a 20 amp gfi breaker in then your receptacle in or a 20 amp breaker in then a gfci receptacle in

then I would just run off the new 20 amp receptacle with a 12 gauge extension cord and splitter for big shop tools, then you can even use the old garage receptacles too for smaller stuff