First day cover subscription services and first day cover dealers are the two major outlets of first day covers on the secondary market (if the issuing postal administration is considered the first) for the collector to obtain his first day covers.
You will realize your own way to collect as you join the world of first day cover collectors. Here are a few of the most accepted ways of collecting these popular covers.
Collect by cachet maker
If you find an artist you like sign up for the first day covers featuring his cachets. Throwing you a curve right off the bat about fdc dealers, be aware that some cachet artists are very popular, with their issues selling out and appreciating in value quickly. It's best to get in on the ground floor and buy from the source at the new issue price. Remember that there are mass produced cachets, which are sold in great numbers, thereby negating any future rarity. But if you're just in it for the fun and not any resale possibilities later, feel free to collect in this way. Just don't expect to recoup any of your investment.
Collect by stamp
This gives you the opportunity of having many different cachets in your collection. If you collect an older stamp issue you obviously won't need a subscription service. But to collect this way you must enjoy the hunt. It takes time -- and money -- to get every cachet issued for a particular stamp issue.
Collect by time period
If there is a period of time that you find philatelically more interesting than the others, you could put together a cover collection focusing on any era, within reason. Remember that you won't find many cacheted first day covers from the early 20s; prior to that time the best you can do is discover an EKU, or "earliest known usage": a first day cover in the days before actual first day covers.
Collect by topic
It would certainly be a worthwile pursuit to put together a first day cover collection featuring your topic on stamp and cachet. Just remember to narrow your topic as much as your interest will allow. You can't possibly put together a complete FDC collection featuring "flowers." But "orchids" is a possibility. The trick is to find a topic that is small enough to be manageable, but large enough to supply a fairly regular flow of stamp issues. Many topics have a glut of issues around anniversaries -- the Pony Express for example -- and then stamps featuring the topic drop off to nearly nothing until the next noteworthy anniversary.
Tip:Look for first day cover dealers who have a deep stock, offering you the greatest possibility in finding items you need for your collection. If you can purchase from several dealers as opposed to scores you will save time, as well as money on the shipping costs that are incurred as more and more the stamp shops close and business is done online.
Of course, you'll feel like a kid in a candy store at a show like the annual Americover show, its raison d'etre, the appreciating and trading of first day covers. A show with many dealers in one place is a real boon to the collector who has multiple items on his want list, so by all means attend shows like Americover if possible. There is so much more going on at these shows (first day cover ceremonies, workshops, cachetmakers bourses) than buying and selling, that after attending a number of these shows you will be an expert at the ins and outs of first day cover collecting and the culture that surrounds it.