If you haven't already discovered www.headforpoints.com, it's an excellent source of info on all things points.

A direct answer to your question would be gift cards. Buy cards for the supermarket you shop at, effectively 'pre-paying' your grocery shop, or for places you know you will be purchasing from in the future - B&Q etc etc. If you buy these cards at Tesco you will also collect 150 Clubcard points for every £50 purchased.

Many others will have many more suggestions!