If you are reading this post you know what you have (Samsung Focus Windows Phone 7 with the default, anemic 8 GB internal storage) and what you want (more storage capacity by adding a microSD card). This is not a detailed how-to. If you do not know how to upgrade your memory and the steps necessary to reset your device to recognize the additional capacity, I suggest finding someone who does rather than going it alone.
There is a lot of confusing information swirling around about Windows Phone 7 certified memory, which cards work and which do not, class 2 vs. class 6, etc. I am writing this because I stumbled across a comment in a forum I frequent that helped me upgrade my Samsung Focus from 8 GB to 24 GB by adding a $40 memory card.
** WARNING: THESE STEPS FACTORY RESET AND ERASE YOUR PHONE **
Step 1: Buy a KOMPUTERBAY 16GB microSD memory card from Amazon for $39. The 32GB card ($89) also works if you need more storage.
Step 2: Turn off the phone
Step 3: Insert the card into the empty slot inside the battery compartment
Step 4: Turn on and reset your phone (Settings->About)
Step 5: Confirm your Total Storage is now 22.12 GB (Settings->About)
Step 6: Redo your settings and configuration and reinstall your apps
Step 7: Fill your new 24 GB Samsung Focus with content