They won't ask you to prove the 30 months residency if your trips outside the US were short. The best way to maintain residency is to take short trips each yr, keep travel records if you want to be careful.
For instance we were out for about 60 days in 5 yrs, so the IO never even looked at the passports. Frequent travellers have been asked for their passport and photocopies were made during the interview and the assumption was that the IO probably went through the stamps to verify travel date.
N400 CSC/San Jose DO
PD : Aug 17 2006
FP date: walk in on Sep 16th/ sched for Sep 20
IV @ San jose do : Feb 15 2007
oath letter: Feb 24th
oath: Mar 21st 2007, became USC