It is possible to hide your affiliate links using 301 redirect. Have a look here for detailed explanation: http://ezinearticles.com/?How-to-Cloak-Affiliate-Links-Using-301-Redirect&id=3005915
Post Affiliate Pro next to direct links mentioned earlier in this thread support also anchor links. If your affiliates will choose "clever" referral id, their links could look like: e.g. referrer id of affiliate is "news", his affiliate link could look like: www.yourdomain.com#news or ref. id is "content", than affiliate link is: www.yourdomain.com#content I'm sure you can find hundreds of good affiliate ids for each web site, which doesn't look like affiliate codes. Next advantage: It is best solution for SEO optimization. Personally I don't have confidence in short redirect links, because I know, that most often is behind such link affiliate link. But if I see clean link just with anchor, I'm quite confident. I think, that anchor affiliate link format is after direct links the best option for you.
I have been using a free generator from http://we.elfdit.com Their links have tracking and you can set them to expire...which makes your products time sensitive and adds scarcity. Works especially well if you have product downloads that you want to secure. You can set your download links to expire and charge your customers an additional fee for a new download link if they lose their product after their initial link expires.
Just create a seperate php file with the code below in it: <? Header( "HTTP/1.1 301 Moved Permanently" ); Header( "Location: yourlink" ); ?> Put your link in and save it as "best-product.php" or whatever you want to call it and direct people to that file.
You can try using php header(refresh to send people to a site your host which will then redirect them to your aff link.
use the Affiliate Hide Plugin which easily creates a mask links for your URLs. It needs a custom field containing affiliate link to work.