Advice with diplomacy; form letters perhaps?

[Baalz]

Yes, this is usually the best way to go, but the other way can work to if he feels he's got a bit of an edge on you and you don't get a response to being the nice guy. If you are fairly evenly matched make sure he knows that even if he manages to squeak out a victory it will be a very costly one. Depending on what you've got available and what he's using you can let him know the alternative to peace is you throwing all your effort into population killing/unrest causing spells, cursing/horror marking/diseasing any particularly good units of his - and of course killing as many as you can as well. Obviously this might backfire as few people respond well to threats, but it can sometimes be a good way to sour your opponents taste for the stalemate.

[Dedas]

Although that could work it is very risky, especially if you want to focus your energy on another foe. The opponent you threatened into peace could suddenly swing about and stab you in the back, or even ally with your enemy to coordinate the thing. Threats between equals almost always lead to a weak peace. Your are better of agreeing (at least in principle). Later on, when you are stronger than him, you declare war or stab him in the back if you feel deceitful.

Always hide your true intention (if you are not roleplaying a honest knight type or similar).

[Gregstrom]

If the opponent has allies, you could perhaps deal with them also. Get them to make overtures on your behalf.

[Digress]

If the opponent has allies, you could perhaps deal with them also. Get them to make overtures on your behalf.

Be careful with that one - sometimes the only thing it results in is their ally sending them a message saying "Lets divide this players empire as follows - I will take provinces A, B and C and you can have the rest ..."