I started working with using index cards for writing ideas similar to the slipcard (Zettelkasten Method). I started working with 3x5's but found they were too small to write ideas on. Now I am using 5x8's. This works well, because I use them in portrait orientation and they fit in the box I got for them. I also got some heavy duty cards that stand up well in the box and are nice to write on.
Here is the first card in the box. Instead of using it as an index, I am using the first one as a list of ideas for what cards could be; this is helpful as it is always the card that is visible in front of the stack.
So far I have made about 15-20 cards, and most are ending up under number 4 (following the system, things under 4 would be 4a, 4b, ...etc) which is where software ideas go. I don't know if this is a great choice compared to always tacking a new idea or concept at the end of the stack. The downside might be that I am always inserting most of the cards at that point as most of my ideas are software ideas. The upside is that I can have them always together so I can flip through them to see what I wanna work on next.