Common sense

There is loads of free advice all over the net with regards to dos and don'ts, so I won't repeat it. I've put some links at the bottom for those of you who are new and not yet familiar.

These are just some of my safety rules and advice:

My basic rule is one you have heard so many times before:
IF IT'S TOO GOOD TO BE TRUE... - don't even bother. You'll be wasting your time

Don't be naive. If 'becoming a star' was so easy, we all would be one! Promise of ££££ for 5 minutes work is a scam. Run.

Be safe! If an audition or filming seems dodgy, don't go (but let them now you can't make it) or take someone with you. I know most of you will say it's not professional. Perhaps, but it feels safer. I have done that on couple of occasions, when audition or filming was in a private house. Nobody minded.
Better safe than sorry. There was one time when I had nobody to come with me, filming was in a private house, far from me and in not so nice area. When I got there I had mixed feelings (curtains were drawn so I couldn't see if there was an actual set). I sat in a car for a while and decided to check if other people are coming. It was a low budged video, and they were looking for quite a few people (according to their listing), so I thought if it's legit, there will be more extras. Once I saw more people knocking on the door and going in I felt safer and assured.

Always check where an audition/filming is. Once you receive a reply with an address - google it. I always use google maps and steer view if possible. Be wary if it's a private house or a park/forest. They do happen and they most likely are legit (as you've read above) but as I said before; better safe than sorry! It never hurts to google the production company if you know the name or any person associated with the project. You might come across valuable information from other extras.

ALWAYS tell someone where you're going. I always text and/or email the address and actual audition listing to my boyfriend and my sister. There are times when you have to have your mobile off for a long period of time and the filming may extend into the night (or very late evening). Your loved ones might get worried if they cannot reach you. Call or text them when you have a break. I also always let them know when I get there to confirm that it's a legit thing.
If the crew takes you to a different location, take your bag and phone with you. No one can expect or demand that your leave your bag elsewhere. Honest people understand that. Most of them will actually advise or insist you to take your valuable belongings with you.

If you're going for filming check the duration if it's not mentioned. They won't be able to tell you the exact time you'll finish, but you have a right to have a rough idea how long they will need you for. If you cannot work beyond certain time, let them know.
If food is not mentioned - ask. The last thing you want is being stuck for hours far away from shops if the food is not provided. This way you can be prepared and bring sandwiches. Loads of low budget productions don't provide food, or by food they mean snacks. If you're unsure just ask. Don't feel bad about it. I always take a bottle of water with me and a small snack if catering is provided. It's just in case if I get peckish before the food is available.

If you drive, always ask about parking. If they don't provide any, check the area (I often use parkopedia: http://en.parkopedia.co.uk/). I don't think it's a good idea to drive to central London for filming, unless there is a free car park provided (also remember about congestion charge during the week).
Even though your expenses might be covered, I sincerely doubt that you will be reimbursed for hours of expensive parking.
It's always a good idea to ask which expenses qualify. They may be limited to travel only like a one day travel card. Just ask, especially if you're working for free. It's hardly worth it if it's going to cost you more than a tube ticket and lunch. Make sure that anything you negotiate/agree on with the production team is sent to you by email. If you agree on something over the phone, simply send them an email to confirm that.

When you see anything like that, especially with UNPAID work:
'major Hollywood star' 'major network' 'big names attached'
Don't keep your hopes up! 99% of them is not true (not all, but most of them) and all they want is get free extras. They will lure you with a promise of great exposure because it's for 'major network'. If you ask them (I did) which one, they will probably say they cannot tell you at the moment but will keep you posted or that there is few networks interested. Basically what it means, they have nobody. They are filming and then trying to sell it to 'major networks'
In general people are very happy to tell you that this and this production is for ITV or Sky Living if it's true. They are also happy to name their famous actors and directors, after all it's hardly a secret!

Links with general advise, dos and don'ts:
http://extras.com/how-to-become/?extra
http://www.thugs4tvfilms.co.uk/tips_and_advice/tips_and_advice.htm

Film glossary
http://www.2020casting.com/artistes-home/artistes-reference/film-glossary