Board Thread:Questions and Answers Forum/@comment-26244589-20150330143907/@comment-24057866-20150330170552

It's all random. Nightfall, strike, and Crucible completion drops are random, engram decryptions are random, mail package rewards are random, and Vanguard Mentor Missive bounty drops are random. Some activities, such as Nightfalls and raids, have higher chances for exotic drops than other activities, but there is nothing that you can "do" to increase those odds further.

Best advice I can give: create one or two more characters of the same class. That way, you can swap your armor between the two characters using your Vault and get rewards for each weekly activity multiple times (once per character).