I narrow it down by trying to focus on a series. For example, I am trying to complete a collection of vintage sport rollies. So far, I've got a 1665 (sea dweller), 6263 (daytona) and 1680 (submariner). I'll pick up a genuine vintage dress rollie to round out the set, such as an early Oyster Perpetual or even Precision. And, all black dial, of course. First and foremost, however, is sticking to quality - not quantity. That's my view, anyway.
Then, I'll try to build a similar set in Panerai. I've already picked up the first piece for that series, a 217. I will pick up a 212 and a Militare with brevet crown.
In short, try to come up with a theme for a series and build your collection around the chosen theme. You might try such categories as chronographs, dress watches, or stick to particular brands. Hope that helps.