What type of bath is it ?
Plastic baths (as many are these days) will need special cleaning, not too abrasive or you'll permanently scratch the surface of the bath. I use a microfibre cloth regularly to keep the bath clean. It uses no abrasives and really gets the bath to sparkle like new.
Enamel baths are usually found in older properties and are made of cast iron (very heavy) with a coating of white enamel. Again if you're too enthusiastic with cleaners you can damage the surface for good. Cif I think is a recommended cleaning solution but if in any doubt read the label on the cleaner first or check here.
http://www.ive.org.uk/enamelcare/enamel-care.h tml