Try avoiding combinations that seem alike. We got the expansion pack not too long ago; avoid factions that play very similarly simply because you tend to run out of options really quick, like the Bears and Pirates combo. Also, avoid playing Ghosts with any faction that's draw heavy like Steampunks/Plants and also factions that rely on returning to the hand like Ninja/Aliens. Like the others said, Wizards get a pass here. If someone is playing Ninjas, avoid factions that rely HEAVILY on actions like Steampunks/Wizards (on a side note; the Steampunk Wizards combo are a force to be reckoned with, so playing them together may be an exception to the statement about Ninjas). In the original pack, it's hard to find a faction that works well with Dinosaurs, but the expansion pack that comes with Steampunks/Bears/Plants/Ghosts give them a little more 'Oomph' but you still might want to avoid playing them with Ghosts.
In a 1-v-1 game, 'You're Pretty Much Borscht' when playing against Zombie Robots unless you're playing the sabotage game with Ninja Aliens, which altogether means your choices are also heavily influenced by your opponent's choices. In reality, the game changes with the choices your opponent makes so choose wisely.
I have yet to play a group game, so I can't make any suggestions there.
Also, note that there's actually another expansion out besides the 'Obligatory Cthulhu Expansion' called 'Science Fiction Double Feature' that comes with Secret Agents/Time Travelers/Cyber Apes/Shapeshifters. Haven't gotten it yet, but I will.
