I have gotten into my mixed voice in the past when working with a vocal teacher. But now I struggle to get there. Any suggestions?
I would recommend doing lip rolls on a scale that uses your mixed voice or right before you break into mixed voice. If you do these daily it will help with your mixed voice. A good time/place to practice these is while you take a hot shower because the moisture in the air will loosen your vocal chords. Never strain your voice as you could cause damage to the vocal chords.
If you are willing to spend money I highly recommend Brett Manning's Mastering Mix program. He is a very professional vocal instructor based out of Nashville. His other programs are very helpful as well.
Take a look at these:
I haven't tried them but they might work for you.