I say that if you are at all concerned about backing up you computer you want to setup a good backup system and have it setup right. I agree with everything listed here. Flash drive is good for moving files back and for not for backup, the do fail from time to time and when they fail there is no recovery.
I would also say that it is a good idea to have a drive at least 2x as big as what you are backing up, however if you are really just trying to have a clone of the data, technically you need at least the same size, but then you will lose the option of using history in time machine once your internal drive fills up.
I am not sure what drive you have internally, but I just ordered one of these early today, because it is on sale.
Newegg.com - Western Digital Elements SE 1TB USB 2.0 Portable Hard Drive This drive is portable and a 1TB, usually MacBook Pro owners care about their backups being portable, but is not required because it will backup only when connected. Right now this drive is $159, it has $30 instant rebate, then if you enter the promo code that is on the page (good until 1/25), it is another $20 off coming out to be $109. It is a good known brand, the older one people were having enclosure issues, if they still have that problem, you can replace that at a later point for $10-$50 depending on options.
f you want wireless backups, the best solution is to go with time capsule or one of the seagate NAS devices. Anything else seems unstable, since apple disable support for every NAS Device except for time capsule.