My first thought is to recommend resisting the sucker shot - that rock star swipe at the ball that's a lot more likely to set sail than land with consistency. The good news is that the initiative is very much on your racquet in that situation, not your opponent's.
If the server's partner is up at the net, you've got the option of picking on that player by going up the line at the alley, lobbing that player on your way to the net, or just testing that player with a zippy return. Not looking to blow anyone's head off there, just push him/her into a more defensive posture.
A deep return is pretty much always a wonderful thing in doubles, but there's also the option of hitting a shorter cross-court return that's more out toward the alley. That placement can really break down an opposing team's ability to cover the court together and it can also force the server to scramble forward and shovel the ball up into your kill zone. Talk to your partner so that he/she knows where you want to put your return. That way, the two of you can more effectively put your opponents in a box.
Even if your deep slice return is getting lobbed back to you a lot, you've still got the initiative there - the other guys are throwing it up and hoping for the best. Be careful to not close in on the net too aggressively if lots of lobs are coming back to your end. You can hit some overheads (hopefully you have that shot in your arsenal) and also cut some short, low slices to again draw the other guys forward if they're playing soft in their own end.
Since there's an extra player on the court in a doubles setting, it's easier for a smart team to cover the width of the court. You can still make them scramble out of position if you work more towards varying your shots between deep and short placement (no, I'm not advocating the drop shot 'n lob snoozefest). Generally, you want to keep your shots low and force the other team to hit up.
