When painting your nails, don't paint your thumbs. Move straight onto your other fingers and apply your base coat, then your colour. If you colour out of the lines, so to speak, simply use your clean thumb nail to tidy up the edges. You can then wipe your thumb nail on a paper towel and continue on painting! This can be repeated with any additional coats, and then you just take some extra care when painting the two thumbs at the end. Finish with your top coat, and you're done!
It takes a little getting used to - I certainly always gravitated towards painting my thumb first and then had to wipe it off. The Vaseline trick works just as well, but this could be a great alternative if you're travelling, as you don't need any extra items. It's a very simple trick but one I haven't seen around, so I hope it comes in handy! Pun intended.