Hi Steffan,
Where are you losing the memory from? As the reported space on your hard drive, as the reported total amount of RAM in your system, or something else?
In activity monitor?
What version of Mac OS or Mac OS X are you using?
What application are you using? Finder? Safari? Something else?
How much total RAM do you have?
How big is your hard drive, and how much of it is free?
If it's Mac OS X anything, at least 15 % of the hard drive should be free for maintenance and self healing purposes.
If the space is really really low and you see space lost when doing anything - well that's normal, the system keeps lots of stuff you do in the cache. The cache size tends to grow from when the system is started until some application or the system is shut down. If this is what you are seeing, free more space - until you have those 15 % free (so 30 GB hd -> 4.5 GB free, 50 GB -> 7.5 GB, 100 GB -> 15 GB etc, as minimum)
Most likely the what to do would include either getting more space, getting another (internal or external) hard drive, or getting more RAM. But before knowing which one is appropriate for you, please tell a bit more. And welcome