I've followed the League (and, less so, Hearthstone) streaming scenes fairly closely over the years, and I think the biggest factor in having streaming success is being there at the start and building your reputation with the game. A very, very large proportion of the popular League streamers have been around since long before the LCS was ever established (and indeed, many good LCS players can barely hold 1k viewers when they do stream). Of the few exceptions, most got there by either getting on very popular teams/being insanely good, or by streaming a lot and working hard to build a viewerbase (like Nightblue, BoxBox, Trick), on a roughly predictable schedule. It helps to make use of Twitter and Youtube to build up as big a "brand" as possible. Good relationships with more popular streamers are useful too.
It helps if you have a "thing" - most commonly being entertaining, or informative, or playing particular weird decks or champions, raging a lot (surprisingly popular - a lot more people like to watch ragers than you'd think). Being female never hurt, nor does having a great voice. If you have music, make sure it isn't too loud and that you can be heard over it. The two big stumbling blocks are building up an initial group of watchers and maintaining a schedule/consistently streaming. There've been some huge streamers who could once command 10k+ viewers with ease (years ago) who've seen their viewerbases dwindle because they didn't maintain consistent shows (or they left Twitch or changed games for whatever reason).
One thing that has put me off several streamers I used to watch often is going in for too much "donation clutter", where half the streaming time is taken up by chat about donators, much of the screen is taken up with donation information, and there are frequent invasive sound effects. Having some acknowledgement is fine, obviously, but make sure it nevers gets out of control.
Hmm, one other thing. I know from several streamers who've discussed revenue (which is of course strictly against the rules, but people used to break them years ago) that the most popular streamers are on far more favourable contracts than the less popular ones, on the whole. Don't expect to get rich unless you end up with a huge fanbase. And don't be too afraid to ask your viewers for donations - I remember one streamer was talking to another (while streaming), much more popular, and, in passing, admitted financial problems. The more popular streamer asked the viewerbase to stump up. At least several hundred dollars, likely a couple of thousand when factoring in future subscription money, were donated that day - largely because people didn't know the streamer was struggling.
