The first and most important area to cover is having a good quality data feed. Gathering some customer reviews can provide users with trust in the seller and can also encourage sales.

http://base.google.com/support/bin/answer.py?hlrm=en-in&answer=25161

This link provides examples to improve your data quality, the best thing you can do is include relevant data in appropriate attributes.

http://base.google.com/support/bin/answer.py?hl=en&answer=78170

If that fails call in the professionals ;)

Optimisation for Google Base Feeds.