What i do is to use the finger thats on the a string to mute the d string. To do this, lay your finger over all strings up to the a string.
The row of I's is your finger.

E----------------------------|
A------I---------------------|
D------I---------------------|
G------I---------------------|
B------I---------------------|
e------I---------------------|

now bend the joint at the top of your finger, such that your finger is lightly touching the D string, and/or the rest of the strings.
Here is a link to a simple picture to help clarify.

http://www.fileforge.com/public/1397/1.gif