1. Don't Slow Play... Versus Tight and/or Passive Opponents
Slow playing is generally a bad idea when your opponents tend to fold a lot and play few hands (are "tight") and when they do play are doing a lot of checking and calling (are "passive"). These players cannot be counted on to bet themselves, and so you need to do the work of building a pot once you've flopped that monster.
After flopping a set, leading out with a bet against such players is often a good strategy. If they are tight and only play strong starting hands, they are more likely to have something of value with which to call your bet. Also, passive players like to call and stick around to see turns and rivers.
Don't bother with trying to "trap" such players by slow playing, as you're more likely to miss value postflop.


2. Don't Slow Play... On "Wet," Coordinated Boards
Let's imagine different flops for your set of eights, say or — boards that are "wet" or coordinated because they potentially give your opponents flush or straight draws. It should be clear why these boards are less favorable for slow playing, and again we can cite a couple of reasons why.
First, you obviously don't want to be checking these flops and giving your opponents a chance at that "free card" on the turn that might complete their draws. On both of these boards, there are a lot of turn cards you wouldn't like to see that could make you less confident about your set.
Also, these are boards that are more likely to have hit your opponents' hands — they may have those draws, or they might have one-pair or even two-pair hands and will therefore be more likely to call if you bet. If they do have, say, just a pair and the turn is a "scare card" that appears to complete a draw, they might shut down after that which means you'll miss value later on in the hand.
(Be smart, though. Some boards are going to be so wet you'll want to put on the brakes — not to slow play, but to control the size of the pot. If you're looking at on the turn, your set of eights isn't such a strong hand anymore.)


3. Don't Slow Play... Versus Multiple Opponents
Finally, as we were noting above, if you flop a set or some other very strong hand in a multi-way pot, you'll more than likely want to bet the hand (or raise someone else's bet) and not slow play.
With multiple opponents there's a greater likelihood someone has a piece of the flop and will give you the action you desire. Multiple opponents also mean more turn and river cards can be potentially bad for you by improving someone to a potentially better hand.
Conclusion

We have two final thoughts regarding slow playing to consider.
First, you'll encounter exceptions to all of the "do's" and "don'ts" listed above — that is, in some rare instances you might actually want to play fast against a certain opponent even though the board is dry, or to slow play strong hands versus multiple opponents. Consider the tips above as rules of thumb to follow, but like all such rules in poker, be ready to break them if circumstances strongly suggest you should.

Finally... don't slow play a big hand just for the sake of being different or to make a "fancy," non-straightforward play. That's the biggest "don't" we can recommend, actually. Only slow play if the situation suggests doing so represents the best avenue toward winning the biggest pot possible. In fact, you'll probably find in most cases the "don'ts" will prevail and it's going to be better to bet your sets and very strong hands postflop.