I think a good approach, depending on your budget of course (since some can afford broader and deeper collections than others) is to concentrate on 2-3 ranges - I do Civil War, Bulge, and D-Day mostly, but also buy small groups of other ranges you like. For example, I love the planes, but I didn't buy the Rommel plane (was tempted though) or Hitler's last fighter; I did buy the Hurricane and Mustang and a couple of sets of figures to go with each. I also have 3 AK vehicles but limited it to that. In addition, I bought the Soviet T-34, a couple of buildings, and 3 sets of figures from that series. It is not an easy line to draw. I have been incredibly tempted by Napoleonics (have 4), American Revolution (have 6), and Egyptian (never pulled the trigger after many Ebay bids), and the Alamo (still none).
Everyone's line is different, but I think buying a lot of some series and a little of others can keep you satisfied and not break the bank. It makes it even harder to draw the line if you look at all the great offerings coming from so many different companies now - K&C, First Legion, Collectors Showcase, and Figarti alone have a slew of awesome WWII stuff, and the new Britains Civil War lines are amazing. Another good approach is to spend a few days thinking about a purchase; if you still want it badly after all that thinking, then it is a good buy. I often decide I will wait or really don't want an item badly enough to spend the money.
