Just ask for the spot already!

You're getting ready for a heavy set on the bench.  Six reps.  You can do it.  You get under the bar, liftoff, first three reps are easy, fourth feels a bit slow, and as you reach the top, you realize you're a bit tired.  Do you really want to drop that weight on your sternum?  Probably not.  You put the bar up and resolve to get a spot next time.  Only you don't.

Why not?  Who knows.  I see guys on the bench all the time whose last rep before they shelve the bar is far too easy.  That last rep should be a close, close thing, which is exactly why you need a spotter - they can take a tiny fraction of that weight and allow you to maximize the value of that set.  In this case, specifically, the adage 'No Pain, No Gain' is spot on - if you aren't pushing hard on that last rep, you aren't making any progress.

Now, let's be honest - when you're putting up 100 pounds, asking the guy on the bench next to you (who's lifting triple that) for a spot can be a little intimidating.  I know, because I've been on both ends - when I started lifting at a gym, my heavy bench set was with 135 pounds.  Ask anyway!  I'm the guy on the other bench now, and I am always happy to spot someone, no matter what they're putting up.  I love seeing the lighter guys working hard and really enjoy helping them succeed.

Last thing - check your weights, and don't go heavier than you can manage for the sake of appearance -   good progress can be made with a 10/6/3/6/3 pyramid (as much as you can lift for that number of reps), so know your limits and push them within the bounds of safety.

Lift hard!