It's almost an inevitability that your bathtub drain will clog at one point or another, but if you're experiencing minor to total clogs on a frequent basis, this is often a sign that there's a bigger problem in your plumbing. You can check on a few of the problems yourself, but you might need to hire a drain cleaning service to help you with particularly difficult clogs.
Lack of Ventilation
Your drains need adequate ventilation in order to drain quickly. Without ventilation, the flow of water can slow down significantly; it's like trying to pour water out of a bottle when air can't get inside. This is often why tub drain stoppers are flat on the top to disperse falling water. But this ventilation also comes from other areas of plumbing, like other drains and outlets. This is a good time to clean out the nearby sink drain that's been giving you problems.
If you simply don't have proper ventilation for your plumbing in the first place, you may need to make some adjustments to your plumbing to provide more air.
Clog Below The Catch
If a partial clog is below the drain catch, you may clean the catch and think you've temporarily fixed the problem, but it won't be long before water is backing up again. This probably means some hair and other gunk is stuck below the catch, probably in the P-trap. There's an easy way to test if the problem is localized to just your tub -- if there's no water from another area, like your sink, toilet or washing machine, it means the clog is before where all the other pipes meet up.
The P-trap is located close enough to the catch so that you can use a simple hand-held snake. A plastic snake with reverse-facing prongs to pull out hair is a good choice. Clogs here are rarely serious enough to require bringing out the heavy equipment. You can use a clog dissolver, but these may be harmful to your pipes, so try a snake first.
Clog In Main Pipe
If you experience frequent clogs that are accompanied by water flooding back into your tub -- such as water draining from your washing machine -- your clog is situated below your drain catches where all the other pipes meet up. The reason you only see it in your tub is typically because your tub is lower than your sinks and toilets. In addition, the reason this results in a frequent clogging is because the water from your tub has enough space to drain partially, but in times of heavy water use or long showers, it will eventually back up.
This is a little more tricky to fix. Attach a drum auger to a cordless drill, then use this to break apart any clogs. This can be difficult because getting past the P-trap is sometimes hard to do with so many bends, and dissolving solutions aren't as potent that far down. If neither of these solutions work, and especially if you cannot access the clog point any other way, it will be time to call in a professional. Click for more information.