This tip only applies to a few specific situations, but in those situations it is very handy.
#1: Assume you have a respec banked, and for some reason you know your character is a bit messed up and you'll probably be respeccing it soon. If you are under level 22, hold on to it. In the meantime, take experimental abilities, ignore the fitness line (since usually you're just loading up on those to get to stamina), and just mess around with things. Then, when you hit level 22, use your respec to sell all your current enhancements (see above), THEN go ahead and work your way up the fitness line (if you want stamina), and fix things.
#2: Related to this is another tip: In some builds, it is very hard to get stamina right at level 20. You often have to hold off until 22 or 24. One of the worst things about this is that it really means you aren't getting the full benefit until 23 or 25 when you can actually get 3 slots in it. Well, holding that respec can let you "cheat the system" a little bit. Wait until you get to the level where you would be taking stamina (if it is 22nd, then you get the benefits of this trick AND the one explained above). When you respec, go ahead and make sure you get stamina by level 20. If this means delaying your travel power until the level you are currently at (say 22 or 24), that's fine (*See Warning below) since you don't need to slot a travel power. Then, since you wedged stamina in at 20, you are able to get it slotted. The net result is that you have all the same powers, but you're able to have that stamina slotted up right away.
NOTE: If you do this, it means you would lose your travel power if you exemplar below 22 or 24. Since it is pretty rare to exemplar that low, this isn't a big deal really. Also, if you are exemping down to help a friend with missions, you can always un-exempt to travel, and re-exempt at the mission door. Just be aware of this if you use this trick.