If you are not a member of the BAA WorldPoints frequent shopping scheme, there are currently two decent bonuses running. If you are a member, you could sign up another family member as long as they have a British Airways Executive Club account to transfer the points.
The Avios promo is here (EDIT: link deleted as now dead). You get 100 points for signing up plus a further 500 when you spend £25 in one day before 30th June.
As I discussed recently, your 500 points is worth 600 Avios. However, if you convert to BMI Diamond Club and then move them to Avios, you will end up with 750 Avios instead.
There is also a Miles & More alternative of this promotion but it seems less interesting as you don’t get the additional 100 sign-up points. Whilst it says you get 500 M&M miles for spending £25, I assume you actually get 500 points and must manually transfer.