Choose Appropriate Content.
This, to me, is the most important thing to remember. Your friends and classmates may be impressed by the realistic textures applied to certain censored body parts, or by how well you penciled a hand-drawn sequence of the famous "lift and separate", or the amazing amount of detail involved in the bloody entrails splayed about in a two-minute grisly, pixelated bloodbath; employers, however, will not be. Nor will they look favorably on your animated tribute to your eternal devotion to whatever mind-alterations happened to get you through semester exams without bursting a vein; odds are they've seen it before, far too many times, and it will not leave a positive impression no matter how well it's animated. There's nothing wrong with having fun with your demo reel, and your animations; the majority of animation is about fun, but when preparing your demo reel, please try to keep it clean and family-friendly.
Professionalism doesn't just include the quality of your work; it includes the maturity to impose the appropriate restraints on the samples that you send to represent yourself. Remember that employers will see your demo reel long before they ever see you, and the content of that reel can cause them to form various impressions of you, both good and bad. Even video game companies that specialize in topless beach volleyball games will look for that professionalism in a reel; they'll be more interested, first, in how well you can maintain a work ethic in a professional environment. You can show them your fascinating technique for rendering bikini tan lines later--at the appropriate time.
Don't Be Afraid to Cut Your Work.
This can apply in two ways; first, don't be afraid to leave work that doesn't measure up off of the demo reel, completely. You don't have to include everything, and if you have weak material that may drag the rest of your reel down, just cut it entirely. If you're just using it as space-filler to meet the minimum length requirements, odds are it'll be pretty obvious and your audience will lose interest quickly.
Second, don't be afraid to cut even your best work. In the course of your school career, you'll probably produce content that could be two or three times the length of your final reel; find the moments in the work that you choose to present that display your skills in the best light, and clip them. You can even cut more than one piece from an animation, and space the parts out depending on where they fit best with your music. There's no need to agonize over just how you're going to fit that dramatic, minute-long scene of a slow camera pan over the same landscape. Cut just enough for viewers to get a good impression of the quality of the work, and move on.
Plan Your Content's Sequence and Timing.
There's a lot to arranging your demo reel; it’s a matter of varying content between types (for example: 2D interspersed with 3D), of building up to a finale, of sequencing to your music. You want to arrange your clips so that your stronger samples support your weaker samples, but you also want to try to tell a story, and can at times cleverly arrange unrelated clips so that they seem to connect. (Your music track can aid in this.)
The best method is to get all of your clips together, and then arrange them in a program like Adobe Premiere. Play with them a bit; shift them around like puzzle pieces with your edited music track attached, until you're sure that you're happy with the arrangement and it has the impact and tells the story that you want. You should start off with something good to hook the audience, but not your best; blend back and forth between your better pieces, and the good-but-not-stunning pieces, so that you're displaying all of your work without losing them in long stretches of mediocrity with the best pieces only at the beginning and the end. Arrange your clips to build up to your finale, which should be your best piece; the last clip will be the final image that stays in the viewers' minds, and a large part of forming their final impression. You want to go out with a "bang", and leave them amazed.
This is where your music will really help you. Once you've got your clips basically arranged, it really helps if you can start tweaking that arrangement, adding or removing frames here and there to shift things just enough so that key moments in the animation combine with key moments in the music track. It's a lot like orchestrating a cinematic soundtrack; if you're using a music track that has a long, thrumming crescendo leading up to a loud cymbal crash, you could time it with something a simple as a character jumping off of a building; the crescendo builds up the fall, and the impact of landing hits with the cymbal crash. Using your soundtrack in that fashion can really make your demo reel "snap", and turn it from a simple arrangement of clips into a dynamic, powerful music video.
