It's the one you don't see that will get you and I suppose you can never guarantee 100% that you'll see everything.
What can you do? Keep eyes outside as much as possibe, get a radar service from someone if you can. Train yourself to actually scan and not just aimlessly look outside, the RAF have a good system for this. Easier said than done.
As for gliders, I'm no expert, but if it's a good gliding day expect to see gliders operating up to cloudbase, whatever height that may be. In areas where there's wave expect to see them a lot higher than you might expect.
Personaly I find gliders quite hard to spot given that most of them are white, first contact is usually when they're turning and you get a glint of sunshine from them. To a certain extent you possibly are relying on the glider seeing you first.
There's a major gliding comp on at Lasham this week, 50+ gliders launching at a time, looks impressive on the radar and even more so when stood underneath watching
WF.