Love my Promarker alcohol pens, and have been collecting them for a few years. They need to be stored horizontally, but the problem with this is that because the ends of the pens aren't marked with the colour or a number, it's difficult to find just the one you want.
I bought a sheet of sticky dots, coloured them as appropriate, and then numbered each pen.
I then did a kind of directory in my notebook, together with colour swatches in the book, and have started to write down favourite blending combinations with examples.
I'm really into colouring at the moment, and already this new system is proving to be perfect! It's a very simple system, and works perfectly for me!