Optimizing a like-cause

Hello,

I'm developing a autocomplete Feature using php and PostgreSQL 8.3.
To fill the autocomplete box I use the following SQL Statement:
select * from _table_ where upper( _field_ ) like '%STRING%';

This SQL Statement takes 900 ms on a Table with 300.000 entries.

What can I do to speed up the Statement? What Index can I set?

Thanks for your Help,
Stefan Sturm