What are some good songs or exercises that could help me improve my muting? There are a lot of songs that when I see mutes, I find them very challenging, and I am unable to play them very fast at all. Now, I know I should start playing the song slowly, but I feel that when I can play the song quickly without the mutes (or substituting the mutes for an extra note), I lose a lot of the motivation I had to learn the song.

Also, any tips or anything else?

(I know this is random, but when I slap my thumb is not parallel to the strings. I have watched many bassists, and most of their thumbs are parallel to the string. When I do the thumbs up, and I start to slap, it is slightly uncomfortable and I find it hard to pop. What do I do?)
A good exercise I can think of is play scales. Play the root regularly, then muted, then the second, then muted etc.

As for your slap technique change it to the parallel way. It's simply the most logical way to slap.
An easy way is just stick with one note and come up with muting patterns - practice with a drum machine or click track so you can work on increasing speed.
For example:
All muted 8th notes.
Every other 8th note muted.
Two muted and one unmuted.
Once you get the technique down with one note, add others on the same string and then start crossing strings.
Thanks for all of your replies.

scawti, this exercise is kind of easy but I will boost up the speed soon.
jazz - this is like killing two birds with one stone (in a good way).
sly - I haven't tried it yet
scawti, this exercise is kind of easy but I will boost up the speed soon.

Start at a speed that is just above your comfort level and experiment with the rhythms. It sounds easy, but it may not be to do it crisp and clear.
err, if you wanna apply it to songs try Blackeyed Blonde, Get Up And Jump or Coffee Shop by RHCP, theyve all got mutes aplenty. So has Tell Me Baby in fact.
Quote by the humanity
You will be the High Lowlord's High Lowlife!

Ibanez EDB 550
Laney RB6