It all depends on what type of camcorder you shot the video with and which project settings you selected when you set up your Premiere Elements project -- and how you got the video from your camcorder into your computer.
If you are using a supported video format and the correct project settings, you should be able to avoid this problem completely (which is much better than trying to fix it).
Lots of reading...
You really don't provide enough information for anyone to help
How to ask a question http://forums.adobe.com/thread/416679
Some specific information that is needed...
Brand/Model Computer (or Brand/Model Motherboard if self-built)
How much system memory you have installed, such as 2Gig or ???
Operating System version, such as Win7 64bit Pro... or whatevevr
-including your security settings, such as are YOU the Administrator
-and have you tried to RIGHT click the program Icon and then select
-the Run as Administrator option (for Windows, not sure about Mac)
Your Firewall settings and brand of anti-virus are you running
Brand/Model graphics card, sush as ATI "xxxx" or nVidia "xxxx"
-or the brand/model graphics chip if on the motherboard
-and the exact driver version for the above graphics card/chip
-and how much video memory you have on your graphics card
Brand/Model sound card, or sound "chip" name on Motherboard
-and the exact driver version for the above sound card/chip
Size(s) and configuration of your hard drive(s)... example below
-and how much FREE space is available on each drive (in Windows
-you RIGHT click the drive letter while using Windows Explorer
-and then select the Properties option to see used/free space)
While in Properties, be sure you have drive indexing set OFF
-for the drive, and for all directories, to improve performance
My 3 hard drives are configured as... (WD = Western Digital)
1 - 320G WD Win7 64bit Pro and all programs
2 - 320G WD Win7 swap file and video projects
3 - 1T WD all video files... read and write
Make sure you have the default Windows hard drive indexing set to OFF for all hard drives and folders
Some/Much of the above are available by going to the Windows
Control Panel and then the Hardware option (Win7 option name)
OR Control Panel--System--Hardware Tab--Device Manager for WinXP
Plus Video-Specific Information http://forums.adobe.com/thread/459220?tstart=0
And, finally, the EXACT type and size of file that is causing you problems
-for pictures, that includes the camera used and the pixel dimensions
Read Bill Hunt on a file type as WRAPPER http://forums.adobe.com/thread/440037?tstart=0
What is a CODEC... a Primer http://forums.adobe.com/thread/546811?tstart=0
What CODEC is INSIDE that file? http://forums.adobe.com/thread/440037?tstart=0
For Mac http://mediainfo.massanti.com/
Read Bill Hunt on editing a VOB/MPG file http://forums.adobe.com/thread/464549?tstart=0
Edit Vob http://premierepro.wikia.com/wiki/FAQ:How_do_I_import_VOB_files_/_edit_a_DVD%3F
Read Hunt on Using MOD/TOD files http://forums.adobe.com/thread/699990?tstart=0
Also, For Windows (not sure about Mac) This is aimed at Premiere Pro, but may help
Work through all of the steps (ideas) listed at http://forums.adobe.com/thread/459220?tstart=0
If your problem isn't fixed after you follow all of the steps, report back with ALL OF THE DETAILS asked for in the FINALLY section at the troubleshooting link
You can once try playing your movie in a third party player like VLC, if this audio-video lag problem still appears in your movie,
try right clicking your clip on timeline in PRE and select unlink audio and video and then you can fine tune the position of audio on the timeline without changing the position of video.
If this audio-video lag problem is not present in other players, then there may be a problem with PRE 9 for some type of file that you are using and you can log a bug for that.
Are you seeing any red lines in the top of the time line above some of the faulty clips?
If so, you have to render the project to see it run correctly in time in preview.
It should be OK on the final product as this renders it when it makes it.
If you have a slow computer, reduct the preview window size to tiny and switch off the thumbnails and audio waveforms in the timeline tracks.
If you have addressed the issues that might be inherent in the Assets themselves, and still have OOS, this ARTICLE will give you tips on how to fix that.
PS - as has been stated, some players do a better job with Audio sync, than others. Do not hesitate to try a couple different ones.