stash a bottle of coke or tonic water somewhere and keep your glass 3/4 full so you can say you're good if someone offers a refill, pick / take a odd coloured straw or mixer / swizzle stick with you and keep it in the glass so you know which one's yours. Keep ice in it too, most people assume a glass with ice and a swizzle stick will have something else in it too so you don't stick out as bad as you would with a bright orange glass of juice all afternoon!
good luck