One of the downsides of a modular synthesizer is the lack of memory banks.
There are times when you have finally found that perfect sound, but need some modules for other patches...
A good (photographic) memory will often do the job in the more simple cases for anyone, but for bigger patches you might need a little reminder sometimes.
To 'store' patches you have a few options;
First, always try to sample the sound you made with a (hard- or software) sampler. Although that often does not catch every aspect of your sound, it is always good to keep what you made in some audio-way. Try sampling it in all its varieties, with your favorite settings and or knob-twists.
Doepfer came up with the nice idea of patch-sheets in the last page of every module's manual.
You can print out these modules, stick them on a bigger sheet (i've even seen some on pinboards) and draw in your knob-settings and patchcables.
The idea is/was nice, but i hardly ever used these sheets. But it can be handy for some...
I also like the online Eurorack Modular Synth Planner. With hundreds of modules from 52 different manufacturers (!) you will be able to configure your patch including a variety of colored patch-cables and save it as a screenshot.
This great site is regularly updated with the latest eurorack modules, but still has a few (small) bugs and imperfections. Follow them on Twitter to get their latest updates.
Another online planner is Stefan Breitenfeld's Modular Planner, also very neat but with much less manufacturers/modules.
My personal favorite way to store my favorite patches is the modern way.
Thanks to my smartphone i always have a digital camera in my pocket.
One or two quick snapshots of my patches is often enough, and only takes a few seconds.