Getting pre flop middle pairs presents you with a dilemma, in as much as your chances of winning without improving on the flop are significantly lower than with high pairs, especially when there are a number of players staying in the pot. The middle pairs are generally recognised to be paired 7s through to 10s, and sometimes jacks. With these pairs the importance of the flop becomes more significant, with a set required to give you a good chance in most cases.
The odds of the flop improving your pair to a set or better are around 7.5 to 1. This equates to a 12% chance you'll improve when starting with a middle pair, hence around 90% of the time you'll end up with the same pair you started with. These stats give you a clear indication that the most sensible option in most cases is to fold, unless you're playing in a short handed game or have set up the game for a bluff, or the pot odds are in your favour and the pot is of sufficient size to warrant the risk of playing on.
Let's assume the pot is big enough for you to consider seeing the flop, and it's going to give you a big enough payoff if you hit the set. The best strategy to follow is to limp in. As an example, you're up against 10 opponents, you've bet $10 which has been matched by everyone else. The pot is now holding $100, and your odds of improving are 7.5 to 1. Playing mid pairs strategy dictates that in most caees you need plenty of opponents to push the odds to a level where it's worth betting. Ideally you'll want someone else to hit valuable cards on the flop when you hit your set, the best case scenario in this situation would be for an opponent to have two pairs. This should ensure some lively betting.
The golden rule is that when you don't hit the set on the flop, then fold. The exception might be if you're holding 8s, and the flop comes 6-3-2. This might encourage another player holding something like A-6 to take a chance on their hand.
Raising is not a good idea with mid pairs. Remember you're trying to disguise your hand with the hope of improving. The aim is to avoid scaring the others away, and ruining your odds in the process.
Playing mid pairs is an easier decision in games with lower numbers of players. In short handed games of less than 5 players, you can be much more relaxed about playing mid pairs. The lower the number of players, the higher the chances that your mid pair will win out in the long run, and the need to hit a set is reduced.