It's very expensive apparently and probably beyond the reach of an average person's wallet for the gains realised.....Cisco routers etc etc
I believe that as Kitz says it does not need to be that expensive, but you will probably need additional equipment and time to study how to set them up. Absolutely on a shoe string:
Two old ADSL modem/routers with ethernet ports (not USB) which can be configured to work in full-bridge mode. If you don't already have these look into ebay and get yourseld two 2WIRE 2700HGV, or SpeedTouch 585v6/v7, which are reported as sync'ing at higher speeds at noisy/long lines. If you live next to the BT exchange you can have any modem - it shouldn't probably make much difference as you will be maxing out on speed anyway.
An old PC (at least Pentium III and preferably with 256MB RAM or more). This will need two network cards, one for each modem, plus a third card for your LAN. Install a firewall/router Linux/FreeBSD distribution - there are many to chose from. The links provided above recommend PFSense. Alternatively, you could use a Linksys with OpenWRT firmware - I know that you can configure it to do load balancing, but I am not sure if it has the CPU and memory power to perform well.
As you will probably have more than one PC on your LAN you could use a third router here between your PC and your LAN PCs as a router, or a multiport switch.
If you hold your nerve when bidding on ebay you could be good to go with as little as £15-20. The idea here is that you can use surplus equipment of your own or from friends. If you want to go professional grade then you could buy a Cisco which you could probably pick up for £50-150, again from ebay, but good samples will cost more.
HTH.
EDIT: some typos & grammar.