All you need is a couple of lead diver's weights (they cost a couple of $$ each from sports store), a hot glue gun and some string. A hot air gun is useful if you've got one.
Step 1) Glue the weights to the side of the hammer. If you have a hot air gun heat the whole thing up so the glue really sticks well (gluing onto a big piece of cold metal isn't ideal...). If you haven't got a hot air gun don't worry, the glue is mainly there to stop the weights from rattling, the real strength is in the string.
You can use other types of glue if you want to (eg. epoxy resin) but hot glue makes it very easy to take apart when it's time for another upgrade - just heat it up and the glue will melt.
Step 2) Bind the weights tightly with some string to make the whole thing really solid.
That's it!
Here's some photos...
My girlfriend's hammer is on the left and is now 5kg (11 pounds). Mine is on the right and is now 7kg (15.4 pounds).
![Image](http://i788.photobucket.com/albums/yy163/shugger1234/hammer1.jpg)
![Image](http://i788.photobucket.com/albums/yy163/shugger1234/hammer2.jpg)