This is where a thesaurus comes in handy. Here's an online one: http://thesaurus.com/browse/think?s=t
The words they list as synonyms for "think" are: believe; anticipate,
assume, be convinced, comprehend, conceive, conclude, consider, credit, deem, determine, envisage, envision, esteem, estimate, expect, fancy, feature, feel, foresee, gather, guess, hold, image, imagine, judge, plan for, presume, project, realize, reckon, regard, see, sense, suppose, surmise, suspect, take, understand, vision, visualize
Of course you should not just blindly substitute synonyms. They don't have exactly the same meaning. But it's a good list to consider. There are also longer phrases that can substitute for "think".
Note that a big difference between many of these words is just the level of certainty that they imply. "I think that you can do it" implies something more than 50/50 but far from certainty. "I hope you can do it" would usually be taken to mean less than 50/50. "I know you can do it" or "I believe you can do it" indicate a high certainty. Etc.
As others note, you can often simply drop the "I think" and state something baldly. Rather than saying, for example, "I think people today are too lazy", you could just state it as a fact: "People today are too lazy." This makes your writing more forceful. In some cases that may be inappropriate, especially if you have doubts yourself. Like, "I think there may be life on other planets" is a reasonable statement. "There is life on other planets" is probably more than you really know. But again, there are many alternatives to "I think". Depending on the context, you could say, "suspect", "theorize", "believe", "presume", "suppose", "hope", etc.