What to do depends on what Mac, Mac OS, video recording software app(s) you are using, and how you are using your Mac.
Here are some things you can try that may remedy your trouble:
(1) Restart Mac, open your recording app only and see if a few-second-long test recording is in sync.
(2) If your trouble is with iMovie, see http://support.apple.com/kb/TS3922
(3) If your trouble is with an internet-connected app or web site, shut down everything, restart your modem, router, an Mac in order and test with NO OTHER connected devices running. If the trouble persists, check your internet speed. (You can use http://speedtest.net or other such tester.) Call your ISP if you aren't getting the speed you need.
(4) Check your disk's available free space. If you don't have a lot of room, free up as much as you can.
(5) Search iSight Discussions for other suggestions from previous topics on this subject.
Here are a couple of example threads:
(6) If your trouble persists, when you post back tell us:
(a) how many seconds is your delay and is that delay the identical between tests and during each test
(b) what Mac and Mac OS you are using
(c) what video recording software app(s) you have tested and which give sync trouble
(d) whether the sync problem is the same regardless of which app plays back the problem test recording
(e) how much RAM and disk free space your Mac has
(f) which (list them individually) of the already offered suggestions you have tried, and
(g) any new information you found while trying these suggestions?
7. We will offer specific suggestions based on the details in your responses.