The battery in the iPad is a Lithium Polymer (LiPo) Lithium Ion battery, using Lithium Cobalt (LiCo) chemistry. These batteries have no memory effect and can be topped up as often as you like. You will not decrease battery life by doing so.
The battery will have ~1,000 charge/discharge cycles in its life, but partial charge/discharges do not count as full cycles, so in practice you may well be able to recharge it >2,000 times, i.e. every day for 6+ years. That's about as long as modern LiIon/LiPo batteries are likely to last anyway, so you need have no fear of causing a problem with daily recharging.
These batteries are better kept topped up rather than empty. If you do happen to run the battery down to empty, this won't damage it as long as you recharge it as soon as possible afterwards.
If you need to store the iPad for an extended period (say 1 month) do so with the battery indicator between 50-80% full.
You'll know if you start getting a problem with the battery - it will no longer hold a full charge. The state-of-charge indicator will drop within ~30 minutes of charging, even when the iPad is switched off. When it drops from 100% to 80% in 30 mins unused, its useful life is effectively over. By that stage you should notice the iPad overheating while charging, which is not a good sign. But that is a long way off for all of us...