Don't forget that Mothercare stores have dedicated babyfeeding rooms. Not as pleasant as going to a cafe, but I have used them sometimes when I couldn't find anywhere else suitable.
Coffee shops such as Cafe Nero etc are good because they often have armchairs which are more comfortable and private to feed your baby on than a normal dining chair.
I always used to take a spare muslin out with me, and if I was feeling a bit exposed I would use it to make sure we were all covered up.
Similarly to some of the clothing tips given above, I found a cardigan useful as I could keep it buttoned up over my tummy whilst lifting up my t-shirt to feed my baby. Wrap-over tops with a vest underneath work in a similar way.
Its unfortunate that many shopping centres combine the baby feeding and changing room, and then don't empty the nappy bins very regularly, and its not a very pleasant environment to sit and feed your baby in for half an hour or so, with the smell of dirty nappies wafting around!
I think its awful that people expect mums to breastfeed babies in public toilets! They wouldn't sit and eat their dinner in the toilet, would they?