before i say this, note that this is only advice for solo shadow leveling, it would be a terrible build for raiding/pvp, even bad for 5 man pugs...
what i did was just gear for pure spirit and spell power.... not really worrying about stamina, only barely worrying about int.
if you gear that way, spirit tap will keep your mana full under almost any situation. you can nonstop pull, right after another, and have a hard time even denting your mana pool much
likes i said, this is only for solo shadow leveling/grinding.... in that scenario, you'll have more than enough mana to kill any one or 2 mobs, and will regen so fast that the small size of your mana pool won't matter. and under this set up, you can be really careless with PW:shield, SWain, so you will still be killing very fast.
you'll wanna have spirit in almost every slot, at least to some extent. random greens of the owl/prophet, and even some pure spirit stuff(which you wouldn't really wanna use under any other circumstance for any class, so it should be really cheap to buy on the AH).
and then try to keep enough spell power to keep things dying fast, but like i said, if you have enough spirit, you can be really careless with SW:P and DP, which will up DPS a fair bit(the inefficiency won't matter)
if you gear this way, it should shoot you up to 60 with no problems soloing. and then past 60(maybe past 70), gear should become alot easier to balance