A recent post on PSAcard.com, "Ten Tips for Building a Collection," brought up several valid points that I often keep in mind when considering a new purchase: Select a collecting theme and stick to it, Ask questions and educate yourself and Keep it fun.
Select a collecting theme and stick to it
This has been very hard for me to do over the years, but having the benefit of hindsight, I always recommend this to beginning collectors. Whether it be jerseys, autographs, a specific team or player, focusing your attention on one common theme will help you better attain your collecting goals. As the PSA article points out, a collection without fences can be incredibly frustrating - you can never have everything.
Ask questions and educate yourself
Just as in school or work, you can never ask too many questions of fellow collectors and sports memorabilia experts when getting serious about collecting. In today's world there are many ways to get information on memorabilia: websites (like The Phillies Collector), discussion forums, memorabilia/card shows and social media outlets such as Twitter or Facebook - use them for all they have to offer! Find those outlets and make sure you have the information you need to make educated purchases.
Keep it fun
Collecting should begin and end with having fun. As a hobby, collecting can provide an escape from the pressures of "the real world" - but, done incorrectly, it can just add to the pile. Sure, it's nice having something that is rare, or "valuable" - but, over the years I have found that the most enjoyable pieces have more sentimental than monetary value. To be a happy collector, having fun is key!