I have a 4 sphere deck that works alright. Not perfect of course but very decent.
1 - 3 heroes from different spheres but I'm convinced that if you balance things right you can work with songs and heroes from only 2 different spheres.
2 - Add up the cost of the cards from each sphere and make it as balanced as possible, it should avoid some resources piling up on one hero (or use songs/Narvi's Belt).
Choose cheap cards, especially allies since you won't be able to get the expensive ones in as fast as with a 2 sphere deck. You can always through Gandalf in but for the spheres you choose try to avoid cards that will take 4 or 5 turns to buy. Once again songs can remedy that a bit but it makes things even more unpredictable. My deck doesn't have a single card that costs more than 3 resources.
Use at least once of the resource producing techniques : Gloin + healing cards, We Are Not Idle (+Lure of Moria) if you play many Dwarves, Thorin, Steward of Gondor, Zigil Miner (with Stargazer), … Choose the one that works with your spheres or, yet again, use songs.
Perhaps not as important but try to get a lot of cards too since you'll want to have a choice of cheap ones at all time. It would be too bad if you got stuck with a handful of cards you can't afford.