Zapple,

IMO the best value is to use your miles for upgrades and use your cash to acquire EQMs. Your target is 1K status. Gold and Platinum are nice, but really do very little for getting you upgraded from Economy, especially on international routes and those with heavy 1K membership (i.e. ORD, IAH, IAD, EWR). You can boost your EQMs by actually buying them. This is however very expensive. There is a FT post that explains how to boost your EQMs with cash.

Another reason for using cash to buy all of your tickets, on United, is the possibility of getting Global Services Status (GS). This is the absolute best way to regularly get complimentary upgrades to Business and First Class. GS status is based on how much you spend on United, not credit card spend. It is widely believed that $50K of actual United and *A purchases is the floor for such consideration.

As a 1K for instance, this year I have upgraded to FC free on 100% of all my domestic flights. My flights have been ORD-SRQ, a route with few regular 1Ks. If on the other hand I was flying ORD-SFO, IAD-SFO or other routes where half the plane are 1Ks, my upgrade sucess would be significantly diminished.

In the end however, there is no way to NOT spend and get regularly upgraded.

Good Luck on your future endeavors.