Playing a tier 2 or 3 demigod and taking risks against enemy teams with average skill will net your about a 40-45% win rate.
Playing a tier 1 dg will add 10%
Playing only stacked games adds 10%
Playing in a premade often adds 15%
Being an uberpro adds 10%
The only way to get above 90% (with more than a few games of course) is to cheat.