It turns out there is a very strong relationship between the number of seats Democrats gain in the House and the change in the popular vote margin from the previous House election. Using a regression based on data from 1932-2010, this year's election can be predicted as shown below.
1. Find House generic ballot polling numbers here.
2. Use the generic ballot number you found to find how many seats Democrats would gain in a typical year, using the black line above or the table here.
3. Adjust for this year's circumstances, which appear to be poorer than average. Your adjustment should keep you within the bounds of the green lines, which are essentially historical limits. I would suggest subtracting 6 - add your suggestion in the comments, and we'll see what the wisdom of the crowds says.
Details
Using the graph.
First, you need to figure out what the popular vote margin for House elections will be (no easy task!), and find that number along the x-axis. (In the last two cycles, polling averages have been pretty accurate, slightly underestimating Democrats.) For example, as of this writing, the polling average show a tie in the generic ballot. The graph above shows that history tells us Democrats should, on average, gain just shy of 25 seats with a tie (black line), although the 95% prediction interval ranges from 2 (lower green line) to 46 (upper green line). In years when conditions are better than average for Democrats, the results should end up above the black line; we'll end up below the black line for years that are tougher than average.
A tie would put chances of taking back the House just south of 50%, if we knew nothing else about this year's elections. However, we do know plenty more information, and circumstances seem less favorable than average for Democrats, implying the final result will end up below the black line.
Adjusting for this year's circumstances.
After finding your general range, then you can start to factor in other things. For example, if you think redistricting gave Democrats a 10-seat structural disadvantage this year, then simply subtract 10 from the number the black line gives you. In this case, the graph would predict (at a popular vote tie) Democratic gains of about 14 seats. If you think circumstances are, for Democrats, the worst they have been in 80 years, then you would look at or just below the lower green line; in the case of a popular vote tie, this would show essentially no change in the number of Democratic seats.
For reasons that are detailed below, I would personally guesstimate that around 5-7 seats should be subtracted from the model's estimate for this year, but I wouldn't bet much on that number. For the pessimistic amongst us, the graph shows us that under the worst circumstances we could not expect Democrats to win the House back unless the popular House vote margin ends up at D+6.
Using the chart to adjust seat-by-seat predictions.
If you have already made your own predictions of the net change in Democratic seats in the House this year, based on seat-by-seat analysis, this graph can still be useful to you. If conditions change suddenly, you can adjust your prediction without waiting for polls from individual races. For example, if you have Democrats gaining 10 seats under conditions of a generic ballot tie, and the generic ballot polls suddenly move to Democrats +5 (don't you wish?), you would add 19 seats to your prediction for a 29 seat total change (oh, I so wish!).
Yes, Democrats could win and still lose.
It is easy to see at a glance how Democrats could win the House popular vote but still not win a majority of seats. In fact, history alone tells us that even if Democrats win the popular House vote by four points, there's still ~10% chance they won't take the House back, and that's without factoring in conditions specific to this year.
Much, much more below, including how well this would have worked in 2010, how the graph was constructed, why it works better than just using popular vote alone, and how to estimate the popular vote without generic ballot polling.