If the battery is older than about 3-4 years, then It May Be Time.

However, first, do the usual - check the connectors are clean and greased, that the cells are topped up (assuming you can). If you have not got one of the no-discharge types, you could try the total discharge trick (H4 bulb wired across the terminals, outside the car, discharge till flat, re-charge). However, not all batteries will allow this. Mike Tempsch is our battery man. Mike?