Technically your internet speed depends on four things:
- The network bandwith as promised on your subscription plan of your ISP
- The available network bandwith of your network
- The available network bandwith of your computer
- The available network bandwith of the site you are connecting to
The lowest bandwith of these four, ususally the subscription plan itself, dictates the current maximal possible connection speed.
The only thing you usually can do is to make more than one connection. Stuff like downthemall uses multiple connections so downloading goes faster.
It may also help to optimize your pc and your network, but they are usually much faster than your isp subscription plan.