My usual advice to beginning watch collectors may have some relevance here as well:
1) As the previous poster just beat me to saying, books are usually cheaper than collectibles - so do your reading first (assuming there are books on Civil War ephemera to read?) An inexpensive book can often save you from an expensive mistake.
2) Strive to know more about what you collect than the typical seller you encounter. Even the playing field as rapidly as you can by becoming knowledgeable, and even expert about the things you collect, by: a) specializing, as has already been said; b) reading everything in your specialty you can get your hands on; and c) getting to know other collectors (i.e., networking, as someone else already said) in your specialty, so you know who to approach for specific kinds of information; who makes the market for the items you want; who has the kinds of items you want and might be willing to sell or trade them some day; who is good to deal with and who to avoid; and who is a competent restorer (if restoration is relevant to your particular field of collectibles).
3) Avoid falling into cliques centered around dominant personalities, which often form in collecting communities. Seek a broad survey of knowledgeable opinions on important questions before forming your own.
4) Buy the best available examples you can afford of whatever it is you collect, and if you can't afford a decent example, strongly consider collecting something else instead. The better examples of any particular collectible tend to hold their value best, they are easiest to resell when you want to turn them back into cash, and they will continue to satisfy you longest as you grow more knowledgeable and discriminating in your collecting.
5) Deals that are too good to be true usually aren't true.
6) Be patient! Not all things, but many things come to those who wait.