1) Contrary to popular belief, creatureless decks actually care MORE about having a good mana curve than creature decks. If a creature deck misses its curve and gets expensive threats instead, it can still win by dropping large threats later. If a creatureless deck misses its curve, it will be quickly destroyed by many small creatures. Relying too heavily on a sweeper, such as Wrath of God, is a bad idea; good opponents will hold creatures in hand if they are winning handily and expect a sweeper, and then cast them in a flurry when you're tapped out after casting the sweeper.
Having a good curve is especially important against aggressive decks that play red (for direct damage) or blue (for Mana Leak). Such decks might be able to ignore a sweeper entirely, by going for direct damage after the sweep or by simply preventing the sweeper from ever resolving.
Find constructive things to do with your early mana. If you are running pure combo deck, cards like Ponder, Preordain, and Telling Time can be vital early plays to ensure you "go off" as soon as possible. If you're going for more of a combo-control, cheap, one-for-one removal, like Lightning Bolt or Tragic Slip, is important to clear out small but efficient creatures so that your opponent must dedicated medium-size creatures to your board sweepers. Don't learn too hard on cheap, one-for-one removal though, or your opponent may counter with tokens or with large creatures.
2) Make sure you have multiple routes to card advantage. Creature-based decks regularly experience card advantage by manuevering into a situation where their big creature effectively cancels two or more smaller creatures; even if the creatures do not actually trade, one card preventing two cards from attacking is virtual card advantage. As a creatureless deck, you will never have this in your favor, and many cards that are balanced by this concept become devastating against you ó for example, the spell Lingering Souls can generate four 1/1 creatures with one card, which is not that a big deal against someone who controls a single 1/2 or larger flier, but devastating against you if left unchecked; sweepers are often circumvented by cards that create tokens more than one time.
Permanents (non-creature ones, I guess) can be good here. For example, a single Night of Soul's Betrayal shuts down both castings of Lingering Souls, plus all other Lingering Souls remaining in the opponent's deck! Many Planeswalkers can shut down multiple creatures with a single card. You also want sweepers and/or pure card drawing to get card advantage. Keep your mind open to "impure" sweepers, such as Arc Trail, which is just as useful against a 2/1 and a 3/2 as a Wrath of God would be but at half the mana cost.
3) Planeswalkers are more powerful against you, since you cannot attack them with creatures as a check against their power. You can remove them three ways: direct removal such as Oblivion Ring, direct damage such as Lightning Bolt, or by running the same planeswalker yourself. The third option is particularly attractive in combination with creature sweepers; since the #1 enemy of planeswalkers are creatures, and you're not running creatures, cards like Wrath of God have a double synergy.