Split pairs of 2s, 3s and 7s against the dealer's up cards from 2-7, otherwise hit. The dealer has a high possibility of going bust when he hits, so you should increase the bet!
The only time you should split a pair of 4s is against the dealer's up card of 5 or 6. In most cases you shouldn't split 4's as you have a total of 8 and can get a good hand with a hit. However, a dealer with 5 or 6 is likely to bust.
Split a pair of 6s if the dealer shows 2 to 6. The dealer's hand gives him high chances of going bust, therefore this is a good opportunity to split the cards.
Split pairs of 8s and Aces. The worst hands are 16, and 12, so you should always split the 8s and Aces for better hands.
Split a pair of 9s against the dealer's 2-6. The dealer has a good chance to go bust, thus you have a better chance of winning if you split.
Split against dealer's 8-9. As you have a total of 18, you will be very likely to go bust if you hit and unlikely to win if you stand, therefore you should split and hope for a good card on one of the hands.
Stand against the dealer's 7, 10 or an Ace. Your hand of 18 is likely to beat the dealer's 7, and it makes no sense to split them now.
Never Split pairs of 5s and 10s. You should not give away your great hand of 10 and a hand of total 20 just to increase the bet.