Everyone loves free advice and most clients will take as much of it as you are willing to dish out. That's why it's vital that you establish quickly in any new business relationship that you are a professional who expects to be paid for your services.
Many beginning consultants are hesitant to do to this believing that they must give the client a 'free sample' of their services, and that demanding a fee too early in the relationship will scare prospects away--rather like asking for a ring on the first date.
However most experienced consultants have found that the only prospects who will be scared away by your mentioning that you get paid for your work are those who had no intention of paying you.
It is appropriate to give a prospect a free consultation-- a meeting in which you can explore their requirements and mutually decide if you can meet each other's needs. State at the beginning of your session that the meeting is free, but then make it clear when the free stuff will run out.
If after you've talked the client needs more help to define his project, explain that you will charge a flat rate for further analysis or bill the client an hourly rate for the same work. Remember: a high-quality analysis of the situation is often the single most valuable contribution you can make to improving the client's systems!
Don't be embarrased to ask for money--as you'll discover should you try to get free services from them, your clients have NO problem doing this themselves! That's why they can afford your services.