In reply to Dave Stelmach:
If one is going to do that sort of thing, it's preferable to state in advance:
'Hi, I'd like to try these gaiters (or whatever) for size. I'm probably not going to buy them here because I can get them cheaper elsewhere. D you mind awfully if I take the piss?'
I'd be a lot happier with that (and would leave them to it sans advice) than the usual
'Can I try these on... great, just what I'm looking for... I'm going to have to think about it'.
We may just be shop staff but we're not stupid. An honest approach will at least gain you some respect.