Most short larp weapons don't need to be properly balanced because after all - they weight almost nothing. However longer weapons, especially two handed swords and other such do benefit from a counterweight. It gives them the feel of a weapon, and allows better control, especially of inertia which is kind of important for a great sword for example, unless you want to put someone down for real.
Note that this is just a testing example - I'm not making a new weapon here. Also - this works with polypropylene pipes, so don't try it for PVC or some other weapon core.
Materials: A handful of nails.
Consider this to be your average larp sword or axe or whatever. The pipe itself obviously has 50/50 weight distribution.
towards the handle, the weight tips towards the front substantially. This can't be allowed, so we will fix it as best as we can.
Now you must hammer it inside the pipe until its firmly lodged in. Once that's done it will be immovable. Since this is a polypropylene pipe, there is no danger of it shattering, the only way to remove the counterweight would be to melt the pipe around it. In this case I've not hammered it in fully - you can get it to fit better and them simply cover it with something.
There you go - now the weight of the weapon is not so disproportionate. Mind you for something heavier, such a counterweight won't be enough but its still better than nothing.