Here are some tips from my own experience:
I encourage my little sis by checking in on her via Facebook. When she posted a status update about being worried about a project due date, I commented that she could call or send a Facebook message to a classmate with a reputation for always being on top of assignments. She did and got the assignment done on time.
I ask her about her grades and help her come up with concrete solutions when she’s struggling with specific subjects. Her brother and I asked her questions to help identify some of her stumbling blocks. She said she felt too tired to do her homework right after school and preferred to do it after dinner. So after dinner is her set homework time now. She was disorganized and kept losing completed assignments before she would turn them in. So we bought her some folders and gave her some tips on organizing. We check in with her regularly to make sure her new system is working.