My son was a late walker (18 months) and I had him in therapy (He has Down syndrome so he was already in therapy) but, 18months is the latest age to begin walking by so there's no worry for you yet.
What I will tell you I did (because a kid with DS walking at 18months is unheard of, usually they are 2 years or older) which was very intensive.
Here's what I did:
1) get on your knees and put a hoola hoop around your waist with your baby inside. She can pull up onto the hoola hoop. Be sure the hoola hoop part she is holding is not above her head as this affects her balance. You want her hands to be as close to her waist as possible as this is where the hands are during normal walking patterns. Then just see if she'll walk around with your like that!
2) get a treadmill and a piano bench or something. Put the bench over the treadmill and hold your baby UNDER her armpits or as close to her waist as you can and her still be able to balance. Have her walk on the treadmill two minutes on, two minutes off for eight minutes until she can do eight minutes at once. This is a long process (by the end of the first week my son did this, he took his first step at 15 months, but then got scared and didn't walk for 3 more months). This will help build leg strength too.
3) Okay, this is a new product I wish I had had with my son. It's called walking wings (www.walkingwings.com) and you can buy it at Babies R Us for less than $30. it's a harness system with two handles off the back for you to hold. The harness holds the child under the armpits leaving their arms free for normal balance and then gives you two straps to hold so you don't have to break your back bending over. I bought it for my daughter, but she started walking before I got a chance to use it. I babysit a 10month old who just LOVES to be in it!
4) if you haven't already, put her toys on the couch or funiture so she has to pull up and cruise to each object. Eventually, she should start letting go and taking steps.
5) If you start to get worried, call Early Childhood Intervention and they will come out and do an assessment and provide therapy if needed.
Let me know if you have any questions.
R.