Your RAM is not a place for 'deleting' as such, and it can be 'used up', but upon restart it is cleaned out and starts fresh, but there are things you can do to improve running performance and efficiency, one as Sami has listed, which will help after many progs leave memory leaks un your RAM, a similar effect can be had by creating a shortcut with this path:
(I won't get into the long of it, but basically it processes the tasks loaded into memory and checks if they are being pointed to, if not it removes them, freeing wasted memory)
Another way to go could be to increase your virtual memory, it will not likely make your computer faster in all aspects, but will give you the options associated with more RAM. This can be done as follows:
right-click my computer, Properties, Advanced tab, under Performance click the Settings button, under advanced of the new dialog, click change under Virtual Memory. The maximum is 4GB or 4x1024MB = 4096MB, but i don't even suggest going that high, try increasing by 10% or so, and see what kind of difference it makes.
Hope this helps,