Why is it that it seems all my inner tubes valves leak over time. Well, except my front tyre, which has stayed inflated for months, whereas even a new inner tube will deflate whilst in the shed...

Is there any tricks or techniques that can be used to stop them leaking once inflated? Theyre only leaking slowly but its still annoying!

buy better tubes?

Most bikes require a bit of a "pump up".. I used to do mine every 2 weeks or so.

I think the seal is only a sleeve of rubber in the valve, its not exactly a high pressure seal.
You should try having tubeless tyres; theyre like sieves  

stick a bit more pressure in it ?

never taken one appart... but I assume its the pressure in the tyre that seals the valve shut

Ive got latex tubes. Even worse than tubeless.