Kabobs to Fajitas - how to make the meat tender
Slice it thinly across the grain.  


That's it.

If it's properly sliced, it'll be tender, as there isn't enough internal structure left for it to resist it falling apart.

Even though it's not technically the correct way to to cook fajitas, I'd go with a mix of Bruce's #1 and 2 : simmer it 'til warmed through in an appropriately flavored liquid (a mix of lime juice and any generic italian salad dressing works well), then drain, and give it some some fast heat to sear.

Of course, this assumes that they're already cooked ... if they're not, then just marinate, cook, and slice (again, against the grain).