You can use a Memory block outside the MATLAB Function block to store your values, then send the output of the Memory block back into the MATLAB Function block. I know this is not pretty, but easy and fast if you only have a couple of variables to worry about. If you have a large number of data to store, try bundle them into an array if possible, so you don't have to deal with a large number of Memory blocks.
Or if you want the memory to be within the MATLAB Function block for tidyness, you can use persistent variables. A limitation of persistent variable though, is that you cannot use it when your Simulink is running variable step solver.
