Once you've discarded your extra cards to the crib and revealed the cut card, it's time for the counting round. Often overlooked by novice players, the counting round is where clever play can be more important than the cards you hold.
Here are some tips on what to lead as the non-dealer in a game of Cribbage:
Don't lead a 5.
With nearly one-third of the deck offering a quick way for your opponent to make 15 for two points, leading a 5 is almost assured to get some sort of ten in response, giving your opponent points for no good reason. There may be extremely rare hands (e.g., 5, 5, 5, A) where leading a 5 is the correct choice, but unless you are a Cribbage expert, you should just avoid leading a 5.
Consider leading with a card that is 4 or lower.
This prevents your opponent from making a fifteen with the next play, and can often set you up to make fifteen. However, low cards can become valuable as the count approaches 31, so you often will not want to use your only low card early on. Aces especially should be saved for later in the counting round.
When leading low cards, lead a 3 if you hold 2, 3. If you have A, 4, then lead a 4.
Your opponent is likely to respond with a ten of some sort, and you can play the other card of the matched set to add up to fifteen for points. If your opponent does not respond with a ten, you can save your valuable low card for later in the counting round.
Tens (and facecards) are a common lead because they are plentiful in the deck. This does not necessarily make them a good lead, but not a bad lead either. If you hold a five in your hand, it slightly reduces the odds that your opponent has a five, making your tens a safer lead.
If choosing between facecards to lead, always lead the card that you have two of.
For example, if your hand is 10, J, J, 5, you should lead the Jack. That way if your opponent makes a pair for two points, you can respond with a pair royale for six points.
This can be extended to a general principle about leading from pairs.
If you have multiple of a card in your hand, it can be a strong lead, because if the opponent makes a pair to gain points, you can respond by gaining even more points. Note that this is especially effective when leading cards value 4 or lower, because the opponent has no way to get points by making a fifteen, so the only point-gaining play against a led 4 is to play another 4. If you hold a third 4 in your hand, you can take advantage of this.
When leading a high card (6-9), try to lead a card with a follow-up plan if your opponent makes fifteen.
For example, if you lead an 8, your opponent will want to play an 7 to make fifteen. You should anticipate this, and be able to either respond with a 6 (for a three-point run), or another 7 (for a two-point pair).