Drinking a lot will help to make the scan clearer, although I agree that it doesn't need to be as much as for the 12 week scan as the baby is higher. Don't forget as well, that the scanners aren't always running to time and the more you've got in your bladder, undoubtedly the longer you'll have to wait!!!
If you are particularly keen to have a girl, then I think it is a really good idea to find out what it's going to be at the scan if at all possible so that you can prepare yourself if it is a boy. I am sure that, after any initial disappointment, even if it is a boy, you will come to terms with it fairly quickly - a healthy baby (fingers crossed) is always a blessing. Good luck.