You may find these links helpful dealing with a slow let down:
Relaxation and Pumping Techniques
Let-down Reflex: Too slow?
I've been having this problem while nursing my LO lately too. She's 8.5 months old and I struggled with a massive oversupply and overactive let down for her first 6 months. As my supply regulated itself my let down slowed, as did the overall flow of milk. She got ticked that her once instant tap of milk was taking so long! So she'll suck a few times, pull off in frustration, grunt a little, suck a little and so on. I've found that visualizing my milk flowing really helps, and trying to relax as much as I can. I also always stroke my little girl's hair when she starts nursing (and many times I'll sing to her), it helps me relax and I think it's a trigger for me that helps signal to my body that it's time to get the milk flowing! And she loves it!
The lactation consultant on this page has some great info too, including this:
"The let down reflex is partially a conditioned response, similar to your mouth watering when you see a picture of your favorite food, you are really responding to your memory of how good that food is, not the picture. So almost anything you would do regularly in association with let down would soon start to signal your breasts to let down. If the deep abdominal breathing is not for you, try using a favorite soothing piece of music, massaging your breasts, softly singing, any activity that helps to relax you. Whatever you choose to do, do it consistantly and make it something you will be able to do anywhere (for example using a warm wet compress on the breast might not be easy in the mall but deep breathing can be done anywhere)."