Good advice on here so far, especially about staying hydrated. However, I do have one tidbit to add that has worked for me. I've hiked/climbed quite a few 13ers/14ers, traversed long ridgelines for several hours between peaks, and have done some ski descents as well.
I use a breathing strategy that has worked for me to alleviate symptoms. When I start to feel that familiar headache, I focus on deep breathing. When I reach the point of a full inhalation, I take a few quick extra breaths in (which puffs out my chest) to get a little more air into my lungs. I suppose it could be psychological, but it seems to work well for me.
A somewhat painful headache or light nasea is fine, but as others have mentioned, if you start noticing more severe headaches or nasuea, or disorientation, it's time to head back down. DO NOT STOP AND REST.
At these altitudes, you'll be fine, as long as you sleep relatively high (~7k-9k) for a couple nights, and take a few day hikes that gain elevation (~2-3k), before giving a 14er a shot. When you get up around 16-18k or higher, thats when you really have to be careful, and AMS becomes a real concern. Conversely, most can do 10k-12k with only feeling extra fatigue and being more out of breath than usual (i.e. no 'real' symptoms), and perhaps a minor headache.