1. Choose something that you like. Drawing something that pleases you will help enormously and give you a sense of satisfaction. If it is a commissioned piece, that isn't always possible, but if you're doing this for practice then make sure you choose something that you will enjoy doing. You'll be looking at it for a long time.
2. Make sure the photo is NO SMALLER than 5 x 7. Personally I won't work from anything smaller than 8 x 10 and have it scanned so that I can enlarge it on my computer screen to see details. It is nearly impossible to work with a tiny photo as you can't see detail or tone correctly, especially in a portrait.
3. Ensure that the photograph is high definition. No fuzzy family holiday snapshots please. They'll drive you insane trying to figure out detail and you will never achieve a good likeness without seeing details.
4. Convert to grayscale to see tonal values. Whether you're working in colour or shades of gray, convert your image to grayscale which will allow you to see the depth of tone that you need to achieve to get a realistic rendering. Most graphic programs have the capability to remove colour and leave you with grays.
5. Choose a simple, uncluttered image. If you want to draw from a photograph you need it to focus on one area or if you have it scanned, to crop out all extra detail and concentrate on one thing. Photos that have too much information in them may be fine for painting sweeps of colour, but aren't good to draw from.
6. Make sure your reference photo is not copyrighted material. Either use your own photographs or if you use those you find in other forums or on the internet, ensure that you ask permission first before you post it or draw it and if permission is given, quote the photographer and source of the image. Don't use celebrity images to draw and post without permission. If in doubt, ask.
7. Choose an image with good contrast lighting. Photographs that are taken with a flash virtually eliminate all shadows from the face by lighting up everything and make it very difficult to draw as everything is flattened. Choose photos taken in natural light with strong constrasts that allow good modelling of features.