+1 on what BrDad said . . . checking out his article "Anatomy of a Great Hide", geocaching.com's hiding guidelines and using the rating system are good places to start.
Also, as he said, hiding a cache can be as rewarding, if not more so, than finding a cache as you read what others say about your cache location and the search for the cache.
First off, it's been said before, but I'll say it again . . . don't worry about the numbers. Normally I would suggest folks do at least 25 caches before they hide their own cache to get a sense of what works and what doesn't, what type of caches they like, what folks place in caches, what folks use for cache containers, etc. . . . and of course you've gone beyond that number -- which honestly is just an arbitrary number that I threw out there.
The very first thing you need to do is find a location for a cache . . . and make sure you get permission for that cache. I suggest thinking about a spot that you like -- maybe it offers a scenic view, there is a historical component, maybe the spot is sentimental to you for some reason or maybe it's just some place that a lot of folks don't know about. I try to hide caches . . . and the types of caches . . . that I like -- i.e. historical based caches (see Before Lizzie Borden), scenic views (I've Found My Thrill), etc.
Picking a good spot is important . . . I can almost guarantee you that if you find a unique spot that offers a view, historical component, challenge, etc. you will get more interesting comments in the logs (on-line and in the log book) than if you hide the cache in a guard rail or lamp post (although to be fair I did a cache by Soapbox. . . in Rumford last weekend that was noteworthy.)
After you find a place for the cache you need to scout around for a good place to hide it. Really think about the hide. Putting a cache right next to a well traveled trail under a teepee made out of sticks or sticks placed end to end is a virtual guarantee that the cache will be MIA in no time . . . however if you placed that same cache off trail a bit in the same manner or if this was done on a trail that rarely receives foot traffic you might be able to keep it around longer. Typical hiding spots that work better (I think) than the stack of wood hide are hollow logs, trees with large roots running on the surface of the ground, downed logs, stumps partially hollowed out, hanging the cache from a branch, etc. Of course there are also many, many devious hides out there . . . if you've ever done any of Laughing Terry's caches or BrDad's caches you'll understand.
In some ways finding a cache hiding spot and determining on how you will hide that cache also depends on the container. I'm not a big fan of micros typically -- there's no room for trade items or Travel Bugs (part of the fun for many geocachers) and quite honestly I think it's more challenging and requires more work usually to do a really sneaky hide with a regular sized cache than a micro . . . I mean no offense, but hiding a typical ammo box in a manner so that it's really challenging to find can be much more of a challenge than hiding a nano. For a good example of a great hide that logs lots of DNFs even though the cache container is small to medium see Ol' 470.
Because of these facts I tend to favor ammo cans -- they're just about indestructible and are very easy to maintain. That said, I've been using some Lock N' Lock containers recently and have had good success with them. Tupperware and cheap Ziplock plasticware do not tend to do well in the outdoor environment. Other containers that seem to work well: bison tubes, 35 mm film canisters, Nalgene bottles . . . and a host of others.
So you've got the spot picked out, know how and where you're going to hide your container (which you've also picked out) . . . now what?
Every cache needs a log book. This can be an actual notebook or just a home-made log . . . or you can even print off a log found at geocaching.com. I also like to put a pencil in with the cache container . . . just in case a geocacher forgot their pen in their vehicle. If the cache is large enough you can then put some trading swag in the container . . . and I always put a stash note in the cache container as well (also found at geocaching.com) in case a non-geocacher stumbles across the cache and wonders what he or she has found.
After that it's time to go on-line and send in the hide to geocaching.com which will review the cache and either approve it, deny it (i.e. too close to another cache, bad location, commer******m, etc.) or ask for clarification.
One other thing to note . . . I never go to the spot and get one set of coords. Instead I try to go there two to three times and then take the average set of coords -- usually the coords are pretty close to each other, but there have been times when the GPSr coords were way, way off . . . and even then I've had a few times when I have transposed numbers and folks have been searching for caches located an entire town or two away from where the cache was located (usually Treazurs-r-fun gets to experience these types of caches.)
Well, that's about it.
"Courage is not the absence of fear, but the realization that there is something more important than fear."
"Death is only one of many ways to die."