Use I-131(may help) and/or I-470.
If you file the I-131 reentry permit form - it appears to help preserve your residency for upto 1 year. There is some ambiguity on this, though, check with a lawyer. The Guide to Naturalization from the INS website also has some examples. You can file a different form for certain special cases which preserves residency for the entire length of time - the form is Form N-470 (Application to Preserve Residency) I think. But the conditions on this form are fairly strict.