This is always a battle when I’m mixing a few songs in an album instead of the entire thing. It’s especially true if I know there’s another mixer who always slams the compressor hard and makes very loud mixes. When mixes are being compared before mastering levels them out, there’s the fear my mixes will sound wimpy by comparison. Of course the style of the album comes into play here. If mixing a rock or dance album which typically has little dynamics you probably will want to make your mixes fit well in that format. One thing I always do before I begin is to find out who will be mastering the album. If I know the mastering engineer and have worked with them in the past, it makes my life easier as I know how my mixes translate to what he’ll do.
But you have to mix in the way that's best for your skills. It's easy to make a loud mix. Anybody can crank a limiter and squash the life out of a mix. But what we need to to is make a great mix, loud or not. One of the most fundamental jobs of a mastering engineer is to make the levels between the songs consistent. Let them handle that.
Which brings me to another point...as often as you can, attend the mastering sessions, even if you’re only mixing a single song. Find out when it’s likely they’ll get to your mix and show up then. I can’t tell you how much I’ve learned about mixing, and the process of what’s done to my work by attending mastering sessions. It’s the only time your work is really judged and you have a chance to talk to the engineer and get feedback. Ask them hard questions. Be ready to hear bad news on occasion and take it to heart. Those are the moments you can really learn and improve. Some engineers may give you a nice word right off the bat, but don’t let it sit there. Let them know you really want to hear their comments so you can improve your skills.