My understanding is that absorption can be increased by making sure you take vitamins and other supplements with food. This also helps if you find supplements make you feel sick right after you take them (a common complaint with iron for example).
As a woman my needs are somewhat different than a guy's would be (mostly the need for increased iron and folic acid) but for what it's worth I take a Women's Multi, a B complex with folic acid, and a vitamin D (I think all Springhill brand).
On backpack trips longer than a couple of days Luke and I both do an emergen-c packet dissolved in 4oz of water every morning. Not sure if it really helps, but seems to keep the energy up and helps keep us from getting sick in the field.