First, you need to create a specific question you will be answering. The question should begin with an interrogatory word (why, when, how, why, etc.) and end in a question mark. "I'm going to write about Cervantes" or "I'm going to write about Galdos's modernity" are not good essay topics. They will confuse you and create a very watered down, imprecise piece of writing. The best thing to do is to come up with a specific question, write it on a cue card, and stick it in front of the computer screen. Then, every sentence you write should be aimed at answering this particular question and not spread out into other topics.
Experienced writers might not need to do that, but beginning academic authors and students really benefit from this strategy. In the courses I teach, I always dedicate at least an hour to practicing the creation of such questions with the students. Say, a student tells me he wants to write about the Cuban Revolution. A 10-page essay on the Cuban Revolution will be a complete disaster, unless the student chooses a specific direction in which he wants to go in his analysis.
The second piece of advice is to come up with a very specific answer you will provide to this question as a result of your research. I have noticed that people who cannot summarize the point of their essay or article in one or two precise sentences are the ones who are not entirely sure about what they are trying to say. Whenever I get bogged down in a sentence or a paragraph, I ask myself, "What am I trying to say here?" And then I try to answer the question aloud. More often than not, I discover that I'm not really sure what it is that I'm trying to say, which is what causes the problem of not knowing how to say it.
One of my colleagues in grad school kept struggling with his dissertation. Whenever we asked him what the dissertation was about, he would launch into a 20-minute response that left everybody confused. Only after he was able to formulate his topic in 2 or 3 concise sentences, did he manage to proceed with the dissertation and finish it.
Feel free to share your own writing tricks in the comments section.