rod, that would be great if we could simply keep rolling a spread that went against us. But I think when you roll a spread that is in the money and has gone against you, it will be a debit, not a credit. For example, the RUT September 850/855 bear call spread is going for a mid price of 3.15 credit. Since RUT closed at 856, the spread is in the money. The same spread for October is going for 2.95, which means that if you rolled it from September to October, you will incur a cost of at least 0.2, probably closer to 0.3. So there will be cost to roll it over to the next month. Now say you roll it, and the RUT comes back down and closes at 848 at October expiration, you should be able to keep that credit.
Now if you get into an OTM call credit spread, say September 860/865, the credit is $2.10. The same spread for October is going for $2.50. So it seems that if a spread is OTM, you get bigger credits the farther in time you go out. The reverse is true with spreads that are ITM; the credits are smaller. Someone like Kim could probably explain why this is so. But I don't think you can simply roll an ITM spread that has gone against you and still get a credit. Someone please correct me if I am wrong.