I've really noticed a bell-curve type pattern with airfare to DLR (and just about anywhere else, for that matter.)
For example, we are flying out Oct. 9. I started looking at airfare back in February, but we weren't sure of our dates at that point, so I couldn't book. Then we figured out exactly when we wanted to go, and the airfare had jumped to around $500 per person (which is nuts for a 1.5 hour flight). We'd resigned ourselves to driving, but I just happened to check the other day, and the prices had dropped to under $200 per person. I was able to get airfare for all three of us for around $500, which was previously the price of one ticket! So it can pay to hold out, but you are taking your chances.
Have you tried setting up any fare alerts? You can do that through sites like kayak.com, it lets you put in your dates and destination, and then it keeps an eye on good fares for you. I'd also suggest trying budget carriers like Jet Blue. We fly with them often, and they are awesome! You get service that is often better than the regular airlines at a discount price.