Trading a bit with the USA (a great supplier of things), Belgium, but ESPECIALLY the Netherlands can make your life *much* easier.
If you get the USA to 200 relations they'll drift naturally to the Axis (slightly), which is really handy if you meant to keep them out of the war until Japan attacks them - or if you're trying to get the fascists to win their elections

.
Having good relations with Belgium and the Netherlands mainly ensures that they don't end up joining the Allies too soon and causing you to have to fight a not phony war with France in the winter of 1939.
Your trade policy as Germany is mainly going to come down to 2 things. Do you intend to run out of resources on purpose? Do you intend to be super efficient with production?
In the first case, you will simply trade as many supplies to the UK, USA, and USSR as they will let you, just keep jacking up the trade to make a lot of money - and then buy as much Metal and Rares as you possibly can Italy, Netherlands, USSR, USA - will supply a bunch. Then build as much IC as you please until 1938 - you will not be able to attain enough resources to feed your industrial machine, and you'll eventually lose IC - but in the mean time you'll have somewhere in the ballpark of 360 IC in 1938 - which should be *more* than enough to build whatever nonsense you want - and for most of 1939 and 1940 you'll have over 400 IC as well until the pump runs dry and you start losing IC to shut down - but by then you should have an army, air force, and Navy more than capable of defeating anybody silly enough to take you on.
If you intend to be more efficient, you will want to limit the amount of supplies you sell to maximize your IC in the here and now, and limit the amount of IC you build so that you wont run yourself out with a minimal amount of trade - the easiest way to achieve this is to build your relations to 200 with ONLY the USA and USSR - and have a minimal amount of relations with the Netherlands (just to keep them from screwing you up down the road) - once you get them to 185 relations, make a very large deal with them - you may want that very large deal to the FIRST deal you make so that they won't haul off and sell all those goodies to somebody else first. Then once you get to 185, cancel that really awful deal you made on jan 1st, and remake it now with a tremendous savings
