Here's a short video guide I made on how to win at caravan, thus getting the Know When To Fold Them and Caravan Master achievement. This guide doesn't give you a guaranteed win but when you play your cards right you will win most of the time. It didn't take me very long to get the achievements. Caravan is more based on strategy rather than luck and the best moment to go for these achievements is in the beginning of the game, in Goodsprings.