When I am offered a draw, I assume that my opponent knows that I may think a bit about it, so I usually look at my opponent, make some facial expression and don't verbally respond. When I refuse, I usually say "OK, let's continue". Unfortunately there are many players, mostly young boys, that don't know how to behave ... sometimes, when I don't respond, they keep asking me. The most funny thing happened to me when my rumanian opponent, not knowing what languages I spoke, offered me the draw in three languages ("Patta?" "Draw?" "Remis?") and then, believing that I still hadn't understood, told GM Istratescu to ask me in English ("Do you want to draw?"). Indeed he was a quite strange guy, all over the game he offered draw something like 12 times, and when at the end I said ("OK then, shall we draw?") he refused, only to re-offer the draw two moves later.
Besides this episode, there are many players that keep offering and offering. After 3 - 4 draw offers, I am likely to became rude, once I said to a 10 year old boy "I said you NO!". Even after that, he didn't give up, he even said "I beg you, I have to do my homework!!". After some moves, I got nervous and I lost a piece, after that he immediately forgot his homework and stopped to offer the draw ... I keep fighting desperately with Queen vs Queen and Bishop, I gave him some 30 checks until he went with his king toward my king and pawns and got checkmated!! That's also why I like chess, because you never get bored!