Don't Work for Free
At some point you're going to be tempted to work for free. That is, a potential client will subtly or not so subtly request a freebie. This could be in the form of recommendations or even a plan of how you would proceed with a certain project. And why shouldn't client's try to get something free? We all do it, after all.
To avoid getting in to the trap of doing work for free, establish quickly that you are a professional and like all professionals you expect to be paid for your knowledge and services. You may be inclined to ignore this advice because you're worried about scaring potential consulting clients away. But experienced consultants know that the only clients that will run away are those that wouldn't have wanted to pay anyway.
What is appropriate is giving the prospect a bit of your time either on the phone or in person. You certainly want to impress upon the person that you're meeting that you know what you're doing and that you can help with the project in question. This initial meeting will certainly contain the exchange of information, but it should by no means turn in to a full-blown design session. One way to ease in to the question of payment is to mention it at the beginning of this meeting. Simply say that you're happy to be there and that this session is free of charge, but future meetings, if there are any, will be billed at whatever your rate is.
What typically happens is that after the initial discussion, the client will want more help defining the project. This is when you reference your original statement of billing them for further work. A good client will realize that the analysis phase is important and should be willing to pay for good work.
Lastly, don't be embarrassed or ashamed of asking for money. Don't worry about whether you'll be perceived as impolite. This is business and your in it to pay your bills. You're not looking to make new friends. And you are by no means behaving strangely. After all, do you think your mechanic would do a complete diagnostic of your car for free? Would a carpenter design a deck for your house and not expect payment?
If you've already got a consulting contract, be sure to read a similar post on working overtime, but not billing for it.