First make sure you have a fairly recent version of Java and Eclipse and the ADT plug-ins. Especially Java.
Then close Eclipse and look at the directory where you installed Eclipse. See a file called eclipse.ini there? Make a backup copy, then open it in a text editor. Look at the last few lines. You'll see something similar to the following:
--launcher.XXMaxPermSize
512m
-showsplash
org.eclipse.platform
--launcher.defaultAction
openFile
-vm
C:/Program Files/Java/jdk1.6.0_24/bin
-vmargs
-Dosgi.requiredJavaVersion=1.5
-Xms512m
-Xmx2g
Don't worry if it doesn't look exactly like that, and don't change anything except the options I'm about to describe. Each option starts on its own line with no white space.
The important parts for performance are the max permanent generation size option, set with these two lines (that must appear before -vmargs):
--launcher.XXMaxPermSize
512m
I recommend 256m if you have 2g or 4g of real memory, or 512m if you have more. Next, the minimum memory option, set with this line (must appear after -vmargs):
-Xms512m
I recommend this be set to the same thing as your max perm size option. Finally, there's the option that controls the total amount of memory that Eclipse will be able to use, set on this line (must appear after -vmargs):
-Xmx2g
I recommend this be set to 1g if you have 2-4g of real memory, and 2g if you have more.
Save the file and restart Eclipse. You may have to adjust the settings up or down to find the optimum value for your system, but the recommendations above are a good starting point.