The bath is a dangerous obstacle to over-come when your mobility is impaired. However, many people can't stand the thought of sacrificing their much loved bath-time. Luckily there are many items out there designed with you in mind- including complete bath replacement systems!
Bath boards are a commonly used tool which sit across the baths edges. They allow for easier entry and exit from the bath, or can be used as a perching area whereby you do not actually enter the bath at all (many people use it for this purpose when they have a 'shower-bath' setup and a normal shower chair \ shower stool will not fit). There are a myriad of types and designs out there; from the basic and budget-conscious plastic types through to the elegant and sightly wooden variety.
Getting into and out of a bath is only half the issue though. Once you are sitting therein the next problem is how to clean easily and effectively. There are lots of options here too including long sponges for handy reaching, toe-washers (the name says it all) and hose adapters for bringing running water directly to you! And make sure when it is time to get out you have placed a non-slip mat down.
The bathroom is the location for 80% of house-hold accidents, many of those are due to errant water creating a slip hazard, so please make sure you address this issue by investing in some bath aids!