Playing ARAMs is one of the first things I try, I don't usually let one bad ARAM game get me down because ARAM so depends on champ select.
If I'm on an ARAM losing streak I go play bots games. There's only two things I like to do in bots games. One is to take champions I know into roles that they are normally never played in (Akali jungle ; Soraka ADC ; LeBlanc support, etc.) ... preferably things that might work out (like Akali jungle and LeBlanc support) although totally troll things like Soraka ADC are fun too. Certainly makes it more challenging to win. (You shouldn't do this if you're not confident that you can beat bots even with the handicap or your team will get mad at you for trolling. Usually this isn't a problem because someone will be on a 30/0/1 rampage on their best champion because they just need a quick and certain win of the day for IP, but you never know when you'll have 2 afks and have to play for real.)
The other thing I use bots for is playing champions that I don't know how to play yet and getting a feel for their kit and mastering their combos. If you have a bunch of recent bundles you probably have a bunch of new champs and skins that you've never played that you could use some time working on their mechanics. For variety take each of them into all five roles.
Spending some time on youtube watching videos on league can help reset your mentality too. I like anklespankin and redmercy, among others. I don't really like watching pro games when I'm on a losing streak... I always end up sympathizing with the losing team (unless I'm watching C9, 'cause they're my favorites I'm always rooting for them).
Worst comes to worst, just take a couple days off, play another game or, y'know, read a book or take a walk or something away from the computer.