Searching for Stock Imagery

I recently started an internship at a digital agency. Every now and again I have to look for stock images.

Does anyone have any tips or tricks for this ? At the moment I am doing one or two word searches with a few filters, but I spend 2-3 hours trawling through complete crap before I find anything half-decent.

Are there any premium sites ? ones with more powerful searches ?

Any help would be greatly appreciated. Thanks.