I have been a Permanent Resident (a 'Green Card' holder) for less than 8 years (thus not a long-term PR), but would now like to abandon my PR status so that I don't have to travel to the US at least once a year just to maintain my PR status. Once I leave the country, I don't see any reason for me to return to the US anytime soon. So far I have filed my tax return yearly and on time.

My question: What would be my tax obligations upon reliquishing my PR status and surrendering my 'Green Card' (I-551) and beyond in the future years?

See IRS form 8854: http://www.irs.gov/pub/irs-pdf/f8854.pdf (instructions: http://www.irs.gov/pub/irs-pdf/i8854.pdf )
And IRS Publication 519, Chapter 4, Expatriation Tax (you'll have to figure out yourself whether it applies to you or not): http://www.irs.gov/pub/irs-pdf/p519.pdf

If you are uncertain on how to apply the rules to your individual circumstances, then you should seek the advice of a competent professional tax advisor. The State Department may have resources available to help you figure it out also.

EDIT: It would also depend on whether you have any US source income in the future. If you no longer have any source income, then you'd have no future obligation (but be sure to properly file form 8854 as may be required).

I would check out http://www.taxplannercpa.com for this and other topics regarding expat taxes. I found them last tax season - they are an office of US CPAs focused exclusively on the taxation of US Citizens living abroad.