Always challenge yourself with your drawing. I draw things that may not appeal to me as much as others. My way is by going on gaia online and doing art requests. Sometimes I find a request that I usually wouldn't draw but the challenge is what makes it worthwile.
If you are low on inspiration then search google images and find a picture you like. I sometimes copy aspects of a picture like the pose or the face but I always change something so that it is unique.
If I am depressed and don't feel like drawing then what I do is I make myself some tea or listen to my favourite music to cheer me up. The trick is to do something to cheer you up and if you still don't feel like drawing then give it a day or two.
If you are stuck on ideas then just start to draw shapes like circles and you may gradually develop an image. Also if you don't know any poses then you can enter a site where models pose and then you can copy the pose. If you see a photo or drawing that appeals to you then just get out a pencil and some paper and start sketching. It takes practise to get great at art and I am still developing my skills. I keep a folder with my artwork and sort it into years. I have 2006, 2007 and now 2008. I only started to get excited about my art 2 years ago and I can already see the progress.
To get better you should search the internet for manga tutorials or buy books to teach you. Be careful what information the books give because I have bought a few books that have conflicted with what the majority were saying. I have 11 manga books and they have all helped me to develop my skills.