I'm steadily declining as well, due to similar unlucky streaks (5-10 losses in a row, 2 or 3 wins, back to losing). However, I do get decent games every now and then and playing mid and jungle tend to help you out if you are decent.
Duo queueing is not as effective as many people think, unless you trust the person you are queueing with and are able to communicate effectively. Friends on skype or that you are LAN'ing with are better than some random from over the internet.
Mid lane can do many things to help out. If you play a solid mid who can push pretty hard and fast (Morgana, Mordekaiser, Cassiopeia, etc) you can ask your jungler for blue, grab their wraiths to put the enemy jungler behind a bit, or gank other lanes or the jungle after pushing your lane.
As a jungler, you can constantly be counterjungling and ganking, putting their team further behind both gold- and experience-wise if done correctly. Good junglers for counterjungling are Lee Sin, Shyvanna, Olaf, Udyr, and others. These champions excel because they are good in 1v1 situations, can take down buffs and camps quickly, and can generally escape with an easier time than most. Good junglers for ganking and snowballing other lanes are Lee, Rammus, Udyr, Maokai, Nautilus, and Alistar. These champions are good for ganking because of the amount of CC they provide for ganks, allowing the lane to claim kills and snowball into winning their lanes.
Also, in solo queue, while individual skill is very important, so are people skills, awareness, and judgement. Know when to go in a fight or stay back. Call barons and dragons at good times. Know when to roam as a group and when to split push. Usually, if you can call a good dragon, baron, or teamfight the first time, your team will trust you and will believe you when you go again.
Be encouraging. By telling your teammates they suck, it demotivates them to do well. When someone tells you how bad you are, does it make you feel like you can win?
Hope I helped.
If you DO want to duo queue or talk more, add me. I'll be more than happy to talk to you or give advice and stuff.