My wan IP changes about 40-50 times a day. It doesn't bother me at all, im never even aware of it unless I monitor it. Its just my ISP.
It suites me perfectly because I can avoid download volume restrictions on certain sites without even trying
If its really bother you, phone your ISP and ask them if they are changing it.
Alternatively, if its your lan IP, check in your router settings how long leases are valid. I have mine set up to change my IP once every two weeks, but I know that it is possible to set it to change far more frequently than that. type in ipconfig/all into a command prompt (go start->run->"cmd" to access the command prompt). What does it say under lease obtained? what does it say under lease expire?