Customising your pets makes it look like you actually care about those little pixels, whether you do or don't.
List of cheap wearbles from Jellyneo
Try and have some kind of theme, for example all christmas items or all flowery items. This will make it seem like you put some effort into customisation than a 2 minute mish-mash of random crap thrown on.
2- User lookups.
Slap a premade onto your userlookup and add a little introduction, such as 'Hi, my name is Neocodexrocks, but you can call me Code. I like cheese toasties and aishas. If you want to be my neofriend, neomail me!'
Perhaps mention any possible 'siblings' or 'friends' with a link if you have more than one main on one IP to make it look like the accounts are not only owned by you.
3- Pet lookups and pages
Most people don't care as much about petpages as they do about the pet's userlookup, so a little attention to that area is wise.
A couple of popular places for premades-
Alternatively, pop 'neopets premade lookups' into google.
Add a sentence or two of your own too. Make sure it is sappy/overdramatic, with lots of smilies. That sort of stuff is food for the legit neopets player, and will make them think of you as one of their own.
An example would be 'Puffyprincesscakiefairy is a princess from Shenkuu who is royalty by day, and a powerful fairy by night!!! She is my ultimate dreamie and she's never UFT/UFA!! I luffs her, but you're welcome to look. Kthnxbai X' Have a bucket nearby for the puking session that will result from writing this kind of prose.
4- Do main-type stuff on your shell
Such as collecting the free NC items they sometimes give out, training your pet for free on it's pet day, dailies, doing a couple of plot steps to get the basic trophy, a handful of games in the DD and GMC, etc. It doesn't have to be the top tier level of participation, but what kind of legit player does't participate in events at all, yet RS's for hours a day?
5- Trophies, stamps and avatars
Get all the clickable avatars and make one of them your active one, instead of default. Default avatar looks really unlegit on a very active account.
Some trophies such as cellblock, snow wars, gogogo or cheat are very easy to get and most can be autoplayed by programs here at codex. Also, optional, are 'tied trophy scores' and can be SS'd without review. The silver or bronze mystery pic and lenny conundrum trophies are very easy to get with the help of codex, and you should have at least 10 easy trophies with no effort.
There's a bunch of 1np stamps from keyquest to chuck into the stamp album to give the appearance of a more well rounded account also.
I estimate all this to take about 10 minutes, 15 tops. TNT may not be fooled (who knows?) but this could at least make players hesistate before reporting what is otherwise an obvious shell account.
6- Neoboards and neofriends
Caution- to prevent loss or impairment of vision, it is advisable to turn off font effects in the 'preferences' dropdown before entering neoboards. This is a fast way of picking up news, makes your account seem legit and a chance to pick up neofriends. From posting a little on the boards you will probably recieve at least one neofriend request a week. Most of these people never want to talk to you again, strangely, so go ahead an accept these if you want.
Fonts- most board users have a font, and the point is to try and fit in with the crowd in order to look legit so you may want to grab a font.
A jelly or snowball themed gallery costs next to nothing, and shows that you do things on neopets that don't involve making np as well.
Again, this is mosly common sense, but common sense ain't so common anymore. As a person with numerous 'brothers' and 'sisters' and 'cousins' and 'best friends' sharing my computer, I've never got frozen despite having 50-60 active accounts on one proxy, the 20 minutes or so to make your accounts look legit could pay. Happy cheating.