I love the causal feel to them! And it's fun finding new fonts and colors that go well together!!
My biggest type is to find fonts that already look handwritten or chalkboard like! (See previous post for list of chalkboard fonts)
I use www.dafont.com for all my fonts! They have the biggest selection of free ones!
It's always good to mix and match fonts! Gives it some life. Mix cursive with sans serif of big bold fonts with hand written ones.
Always try to stick to using light pastel colors! They look more natural!
When adding a picture to a chalkboard background I recommend using a feathered effect because it takes away the sharp edges allowing it to look almost as if someone colored it onto the board. Or of course you could use a frame and maybe give it a drop shadow effect to make it look like it's hanging on the board!
Check out my Etsy shop for some of my chalkboard listings! :)