When you are choosing a conduit for inner-ducting, you have to keep in mind that you will be fishing the inner-duct through a larger diameter duct. Thus, you need a material that is flexible enough to follow any curves or contours in the larger duct. For many people, the material of choice for inner ducting is high density polyethylene, or HDPE. Be aware that even when choosing HDPE pipe, there are still some features you have to take into consideration.
Corrugated or Not
If you want a pipe that will stand up to some pressure, then corrugated pipe is the way to go. The ribs in the pipe give it a little more crush resistance than smooth pipe. The corrugations also make it more flexible. On the other hand, if you don't have to worry as much about going around sharp corners or having your pipes stand up to rough wear, the ribs in corrugated pipe can catch on pipes as you try to fish more pipes through afterward. So if you are fishing many HDPE pipes through a larger duct, smooth pipes might be the better way to go.
If you have to run cables through a ceiling, you have to make sure that the cables are treated in a way that makes them fire resistant. HDPE pipe will melt, smoke, burn, and release noxious fumes. Thus, if you have to run cable through a ceiling, you should look for plenum pipe to use as conduit.
While HDPE pipe will come in rolls of up to a couple of hundred of feet in length, you still might have to couple two or more sections of pipe together. If this is the case, then you will need to use couplers. When choosing a coupler, make sure you choose something that will not take up too much of the space inside the pipe. it should also not have edges that could catch cables as you try to push them through the pipe.
If you are having a hard time deciding what sort of HDPE pipe to use for your project, and/or you want to know what your options are for using couplers, you should talk to an HDPE pipe supply company. They should be able to advise you on what pipe to use, what the easiest way to run the pipe is, and so on. The key to a successful project is to start with a good plan. As long as you have the right HDPE pipe and couplers, you should have no problem with inner-ducting.