Step 1: Prepping the Laptop
Firstly, you really need to spend the time to make the power connector robust and reliable. Many of the connectors available from farnell/RS will either be rather expensive or won't have the right power rating. The other problem I found was getting one which was not by far too long. I ended up going to a maplin store so that I could actually see them and hold them up to my computer.
Once you have made up the connector, solder the wire as you would expect (red to the middle pin and black to the outer rim of the barrel) I found putting a bit of sugru at the end of the wires and into the end of the barrel jack quite helpful (clearly not at the metal end). This helps to prevent the barrel for being able to rotate free once the sugru has set, and will cause less strain on your wires.
Step 2: Moulding the Sugru
Now you have prepped the wire you are ready to push it into your laptop. Make sure there is no bare wire, loose bit, or anything that can short out or cause damage at the end of the barrel jack.
When pushing in, don't push with all your force! You really don't want to damage the cables in any way. I found you that I needed to push mine in enough for it to stay there (as in not come out due to flex in the lead).
I then used a knife to scratch away some of the varnish around the where the sugru was going to go. The sugru does a very good job at sticking to the varnish, but I found that the varnish did not do a very good job at sticking to the laptop.
I then applied liberal sugru (as you can tell colour does not matter to me and I thought the yellow looked quite striking).
I found using a tiny bit of water (like licking your fingers but you clearly don't want to be ingesting sugru) was helpful for getting a smooth finish.
Now, BE PATIENT!!! Wait for the sugru to properly set before you start messing around with it. If you don't then the flexibility in the barrel plastic is often enough for it not to get a very good contact.
Anyway, hope these tips help someone...
Step 3: Few Extra Fixes
Because my laptop is old it has started forming a few cracks, sugru is great for fixing that (this time I thought black would look soo much nicer).