I'm not a fantastic cosplayer, but I still love it! We don't have many big anime events around here (i'm involved in getting one here, I'm on the main and original staff of QC Anime-ZING... it's planned for 2010, we're a baby con that's just starting, haha) so I only cosplay to small events that are in my area- store events at FYE mainly, but I LOVE cosplay!
L from Death Note is incredibly easy to do... He was my first 'official' cosplay, meaning he was the first cosplay I did with effort and wore to an anime event. Really, any of the Death Note characters (human, I mean) are. My sister cosplays as Misa Amane from the same anime, and everyone always tells us we look great. Technically, mine is Crossplaying, but still... haha.
I think acting like the character you're cosplaying as is a big YES, especially for pictures. I'm not saying u have to act like them that whole day, but know the character, practice poses for pictures that are in-character, etc.
For example- L stands/sits weird- all hunched over, and if he's sitting, his knees are pulled to his chest and he's technically crouching. And he's ALWAYS eating sweets and always looks shy- doesn't really smile wide or anything. So I take that knowledge and use it for pictures or for when I'm in a cosplay contest- my L cosplay won 3rd place in a contest in which the attendees picked their fave cosplayer, factoring in costume accuracy, hair accuracy, poses, etc. The whole room started chanting "L" over and over again when it was time to decide the final winner. It was pretty awesome. :D
Another thing I LOVE seeing on cosplayers is WIGS. If your hair isn't the same style or color and you can't color/style it (i have long black hair, and there was NO way I was cutting it for L) wigs are the answer! Pre-styled ones can be found for almost any character online, but they can be pricey (well...all cosplay is pricey, honestly. it's an expensive hobby and breaks my bank more often than not, but I still love it!). I like to buy wigs that aren't styled yet and then style them on my own because it's often cheaper that way. BUT if you choose this route for a wig, BE CAREFUL! Wigs aren't really like styling real hair- they can melt if exposed to too high of heat (like really hot curlers or straighteners) and some products don't work well on the fibers. I researched styling wigs and asked for tips from other cosplayers. For all my wig tips, I went to
www.cosplay.com. they're amazing people there. :D
If you're crossplaying (a girl cosplaying as a guy- like me cosplaying as L, or vice versa) you may want to consider binding, if you're a girl. Guys can 'tuck'- for more info on that, go to cosplay.com- it gets graphic in details. I have a rather large chest size, so I bind when I crossplay- I use athletic tape and an ace bandage. It's just a thought- L doesn't have breasts so I bind. :) Cosplay.com has a wonderful thread detailing how to bind, and that's where I got my info for it. It works really well. :D
I hope this helps u at least a little... :)