Rule #1 -- Find a reason/theme for your mix. Do you want a workout mix, a love dedication mix, or just a drivin’ around summer night mix? The reason is what drives the emotions in a mix...even if that reason is just “songs I like right now”. I used to have a mix called “songs I’m ashamed to like” before I learned to be ashamed of nothing...mostly.
Rule #2 -- Decide a length for your mix. If you’re not, as you probably aren’t, using a cassette or a CD, mp3 playlists can be daunting without an expiration within a reasonable listening duration, and you need one especially if you are intending to give your mix to others. One can’t expect a person to listen to something longer than an hour and a half, in my experience. If you’re making a mix for an exercise routine, make it as long as your routine. If you’re making a makeout mix, consider the phase of your relationship and time it accordingly. Of course, if you ARE making a mix for cassette or CD, your time is already set and working within the confines of sides on a cassette is a finesse that will be appreciated by your mix’s lucky beneficiary, if they have ANY sense whatsoever.
Rule #3 -- Pick a song order (if an order doesn’t show itself to you organically). There are many ways to do this. If you want to make it really easy, order the songs by length (shortest to longest or vise versa) or chronology, or even by alphabet. If you want something a bit more evocative, I’ve found that it helps to start with excitement: high beats per minute and something joyous/rawk-us. Then, make track 2 even MORE exciting, take it down a notch on track 3, and continue to play with emotions throughout with at least a 4 or 5 track wind-down at the end...personally, I think a really good mix ends in a nap, but only if they sleep when you tell them to sleep, gosh darn it!
Disregard all of the above, and any other rules on mixes if they don’t suit you. MAKE MIXES FOR YOU, and no one else. Make them to your liking, not only because people are exceedingly under-appreciative of mixes in general, but also, if you aren’t putting your heart (and freakin’ rhythm ‘n’ soul!) into your mixes, they REALLY won’t be appreciated, so do what you want always and worry not how they will be received/perceived. I always make mixes for me and if they involve thoughts of another, I give that person a copy and hope they listen/love it as much as I do.
You have it within you: go forth and follow your rockin’ psyche, mixters and mixtresses!