A writing journal should consist not only of your writing, but also of any stray ideas or interesting facts you collect throughout the day. Keeping a small notebook or a digital recorder in your pocket at all times may sound cheesy, but if you've ever had a burst of inspiration at bedtime that you'd forgotten entirely by breakfast, you know how important it is to write things down as you think of them. Transferring notes to your writing journal will give you more time to process them and, if all goes well, turn them into longer pieces.
If you are dissatisfied with a piece or so satisfied that you think you might have lost your sense of objectivity, writing a reflective essay can help you to determine what needs to be improved, added or left alone. Writing down your thoughts can help you to think even more critically about your piece and its relative merits and shortcomings. Note that it may be a good idea to leave the piece alone for two or three weeks to be sure you are approaching it with new eyes.
Once you've mastered one form, or at least gotten tired of it, branch out into other areas of writing, especially those you've never been interested in before. A poet should try writing an extended essay and a satirist should try writing one of the sappy romance stories they despise. Writing outside your comfort zone will diversify your skill set and expose you to new ideas and sentence constructions alike. Don't feel obligated to finish a project like this, especially if you choose a genre you loathe, but at least try it. You may find yourself pleasantly surprised with the results.
You'll be amazed at how a simple prompt can inspire pages of writing. For poetry, make a circle with your thumb and forefinger and hold it in front of you. Write a poem describing what you see in the circle. Write a short story inspired by a piece of visual art, or write a personal essay about a negative experience that made you stronger.
There are also a number of free websites that provide writing prompts.