The advice would depend on a couple factors. Is this for a group of people (say, for a party, or your circle of friends), or is the mix for one specific person (a crush, a friend, yourself)?
If it’s for a group of people, know your audience and I usually err on the side of mostly singles with a few good deep cuts thrown in just so it doesn’t sound like all Top 40 radio. If everyone in the group is in their mid-20s, it’s pretty safe to assume they like music from the very early 00s best as *most* people consider the music released while they were in high school to be the best music ever.
If the mix is for a specific person (crush or friend), I usually try for half songs they would be familiar with, and half songs I don’t think they know, but think they would enjoy. Pink Floyd is one of my favorite bands, but I wouldn’t want someone giving me a mix of all Floyd songs. I would, however, probably enjoy some Bowie and Radiohead on a mix, because there is a lot of crossover between fans of Floyd and fans of Bowie and Radiohead.
Good mixes tend to have an overall theme, for most people it’d be pretty jarring to hear New Kids on the Block followed by Nine Inch Nails. If your theme is nostalgic pop, stick with NKOTB and maybe some Madonna, Duran Duran, and INXS. I could see those four artists sitting comfortably next to each other on a late 80s/early 90s pop mix. (I could also see some early NIN (Pretty Hate Machine-era) sitting next to INXS or Duran Duran on a mix well, so there are different ranges to work within.)
Or forget genre (within reason) and maybe your theme is creepy love songs (in which case NKOTB’s “I’ll Be Loving You (Forever)” could probably sit next to NIN’s “Closer” on the same mix). Or all songs about capitalism (Floyd’s “Money”, John Lennon’s “Working Class Hero”, Dire Strait’s “Money for Nothing”, Paul McCartney’s “The Pound Is Sinking”). Or - and this is a mix I’ve actually made - a mix of dance songs featuring amazing guitars (“Black Cat” by Janet Jackson, “Mony Mony” by Billy Idol, “Like A Prayer (7” Version)” (which featured a new guitar solo by none other than Prince!) by Madonna…)
Of course, now we’re getting into mixes you’re making for yourself. Which is also totally cool. I love curating mixes, even if I’m the only one who’s going to listen to them. What better way to know what your listener will enjoy, right? =) Mixes for yourself should be honest. You might love some songs, or feel some songs are “classics”, but if you’re not feeling “Stairway To Heaven” right now, don’t feel obligated to include it on your Best Solos of All-Time mix…
I hope that helps. I just realized these were all content tips and not really technical tips. If you meant technical tips, very briefly, make sure you adjust the volume levels of each song so your listener doesn’t have to touch their volume controls. You can do this in free software like iTunes, or in more professional audio software like Cubase or Logic. And in fact, with multitrack software like Cubase and Logic, you could even edit your own segues/fades between songs, which I did for all three of the DFTBA Records compilation mixes. That’s always fun. Just use your ears for best results there.