There's no need to offer a huge explanation straight away. Just say 'oh I'm looking for $X' and smile. Most of the time people don't push it further.
If they ask for more info, just repeat & say 'well I'd like it to be noted that for this position I'd be looking for $x'
It can feel really awkward but remember it's equally awkward for the person asking your salary. Try & keep it light hearted with short answers, you only only have to be slightly more persistent than they are to keep the number secret.