I've worked tables, and nothing was worse than doing your job and being stiffed for a gratuity afterward. Kinda makes one sour pretty quickly and you only do the minimum and hope for the best - and feel kind of bad afterward when you DO the minimum and they then hit you pretty well.
One day a couple tipped me in advance and I made sure I paid them a bit of extra attention while they were there. I mentioned, at the end of their meal, that what they did was pretty unusual and they asked me what my thoughts were when they tipped me and then explained what I did in my first posting. It felt good, and was a bit humbling, to be trusted to do what was right for what I received; rather than performing a service and hoping I didn't get stiffed.
I've since tried to do the same when I'm in a 'tipping situation', have only been disappointed once, and have hopefully spread the practice - not ONE 'tippee' has initally failed to ask why I'm doing it, and I've told them what that couple told me.
Most waitresses I know would be insulted if you tipped them upfront. You might as well be throwing a bone.