Are you meaning a perch in the cage or a perch stand (t-stand, etc)? If you are meaning a stand then the best way would be as parakeet suggested -- if her not going near the perch is fear related. Show interest in the perch yourself, lean against it, touch it, maybe even have a little snack (especially if your bird is usually interested in your food) while leaning against the stand. After a few days of this while holding your bird and speaking calmly to it go near the stand. Don't try to make her step onto the stand or even touch it at first just go near it. Go closer the next day and the following until you are leaning against or standing against the perch without making her get onto the perch. Make sure you are speaking calmly during these times even offer a special treat to keep her calm. After she is used to you being at the perch see if she will step onto it for a treat. Hold the treat on one side of the perch so she will have to at least lean over the perch to get to the treat. Keep doing this until she will actually step onto the perch for the treat then praise her and take her off the stand to go do something else. The first time take her back off almost immediately after she steps onto it, then each time make it a little longer until she seems used to the perch. Then you can try moving away from the perch gradually.
If it is not fear related and she will get onto the perch but just doesn't want to stay there you may be inadvertantly reinforcing her actions to leave the perch. Do you automatically pick her up when she leaves it or go hunt her down wherever she is in the room? Does she get more attention because she is not on the perch. Does the perch have anything to keep her entertained? Toys, bells, treats etc. If she is bored she is bound to go exploring to find something more interesting to do so try to make the perch interesting with things you know she likes and reinforce when she does stay on th perch by giving her praise, attention and treats.