Short answer, it's a real bitch. Engrams drop in a bracket according to your current light level, so the easiest way is to make sure when you decrypt engrams that your absolute highest gear is equipped. As you decode each one, check if it's higher and if it is, equip that new item before decrypting anything else. After everything is decrypted you can go ahead and infuse the high blues into the good but low level purples. Also if you pick up any exotic engrams, vault them until you reach 300. Sub 300 they will all decrypt at 290, but once you're over 300 there's a good chance of them decrypting at 310. Other than that, make sure you're doing events like iron banner, where you have a chance at some 320 gear. I was stuck at 295 for a month, and 3 days of iron banner bumped me up to 306. Once you're over 290 you can look at raids too. Doing the challenges on nornal mode guarantee 310 gear! You could jump 5 or more light in 1 raid. That's everything I can think of! Feel free to add me and we can do some raids!