Yeah, it's tricky, but not impossible, to find good deals on GTS.
If you're looking for a pokemon and get nothing but unreasonable trades (Seriously, you can't even legitimately ACQUIRE a Level 1-10 Reshiram, people!) try searching by each of the level tiers. If you still fill up with trades, try searching each tier for males and females. There's usually one or two reasonable trades out there, but for some reason GTS only lets you see that first handful.
Failing that, if you make a reasonable offer yourself, you'll usually get the trade in a day or two.