I've had a year that I can't wait to get over with. Lot's of stuffs happened. Some good, some bad... I reckon, for about 80% of my workouts for the last 6-9 months, i've felt like crap. I feel i've actually "adapted" to being able to train with not enough calories in me, or not enough sleep, or just feeling completely run down.
-cue worlds smallest violin, playing the worlds saddest song-
Anyway, I think there's a few different situations and approaches. If you're ill but not ill enough to give the gym a miss, then easing off, using less weight and just getting "something" done without running yourself into the ground would be my recommended common sense approach.
If you just generally feel like crap it's going to be from physical stress from training/not eating or resting enough. Or, emotional stress from other issues out with the gym/diet. In this case, I've noticed 2 main things will happen. You can go in there, warm up, do a few work up sets, and end up smashing it. It's not really the norm but it can happen. In some/a lot of cases, though, you'll just be weak, plain and simple. Weights you can hit for 5 reps will go up slow as hell. In this case I think it's good to try and get your reps in. If you just feel REALLY crap then i'll just keep the reps high, the weight light, and pretty much aim to get pumped up. It's also a good time to get lots of "healthy stuff" done. If you do this it normally makes you leave the gym feeling great, physically. You'll still be mad that you were weak, but it's far better than just bailing out then basking in your own selfdisgust
It's amazing what you can do with a bit of anger though. I'm fed up feeling like crap when I train so, when I experience it, I get really mad. It's these days I'll normally be training and i'll get the most stupid questions/requests from the most annoying members in the gym (I have very limited time to train, so I value every minute). Normally I can shrug it off but sometimes it just feeds the aggression. In these days I normally either smash it or, i'll compeltely run myself into the ground and make things worse in the long run
What I try and remind myself is - What do I do with clients who feel like crap? I can tell with a regular client whether they're stressed in a way that'll help the workout, or stressed in a way that I need to reel them in a little. If I know I need to reel them in, then my goal is simply to have them leave the gym in a much better mood than they arrived. Basically I try and take my own advice but don't do it very well.