1) Your profile image is key.
When I go to a portfolio website like Purpleport or ModelMayhem to look for models I’m confronted with hundreds and hundreds of faces in a row. All I see is your profile picture and name. There aren’t too many photographers who have the time to go through and click every single profile so you need to make sure you lure as many in as possible with your profile picture. Choose a simple close up of your face. It’s hard / impossible to see what you look like in full body shots / conceptual shots so keep it simple and make sure it’s a strong image.
2) Quality not quantity.
If a site allows you to upload 12 / 15 / 20 images don't feel that you absolutely have to. Although it's important to show a varied and high quality portfolio a poor image will do more damage than it's worth. Whenever you upload a photo ask yourself whether that image is really benefiting your portfolio or whether you're just putting it there because you can. Is it a high quality image? Does it show your variety? Does it actually add anything to your portfolio? If not then leave it out.
3) Don't be a diva.
Unless you are extremely lucky it's unlikely that you're living a life of luxury on the earnings you make from your modelling. You're more likely working in a crappy job trying to get into modelling on the side. This is fine! Photographers don't expect you to all be Cara's or Kate's. When you write your profile don't pretend to be something you're not. The top photographers know how to source models and the less experienced ones are more likely to be put off by your grandiose statements than they are to be impressed. Be honest about your experience level and don't try and be something you're not.
4) What you write is as important as the photos you post (Well.. nearly.)
I'd hope as a model you realise that there is more to it than being pretty and pouting in front of a camera. A good model needs a level of intelligence so that she can follow commands, come up with her own ideas and in general not be clueless on set. If I see a poorly written profile with bad spelling, grammar or jokes I switch off instantly. Your portfolio is your CV and just like a CV it's of absolute key importance that you ensure that it is well written.
5) Don't waste photographers time.
Don't click the checkbox that says you'll do lingerie / topless if you won't. Don't reply to photographers emails with vague / stupid responses. Be a professional. It's fine to have limits or areas you specialise in but you need to make sure that these are clear and correct on your profile page. Make sure your portfolio accurately states your age, height, measurements and shoot styles. Don't lie. If I can see you're lying about one thing then I can't trust anything else you say and I am unlikely to approach you.