Oh god, don't actually try that please anyone!!
With bigger engine (360 and bigger) then leaning whilst taxiing is definately recommended. (It's certainly in the POH most of the time.) But with little engines (O200 etc.) then they shouldn't foul up as readily. At least not at low Density Alt's.
If you have a prolonged period at low power, then fouling is a distinct possibility on any engine.
To clear it, follow the advice already given, increase RPM and lean the engine for a short period. If this doesn't clear it, try again, but be aware that mag problems are sometimes caused by other issues that require an engineer to clear them.
For example I saw an FI sitting at a hold point the other day with an engine back firing, popping and generally sounding like it was about to collapse as he s*dded about trying to clear it, when there was really no doubt that it wasn't a case of fouling but something far more serious. The look of pain on the engineer's faces was something to behold.
So if it's really bad and it won't clear, give up before you do a lot more serious damage.