1. Don't learn similar characters together.
According to interference theory in Psychology, learning similar things together can cause those items to interfere with each other in your memory. So when you are shown, for example, "天", you may recall the meaning of the similar item "夫" if you learned these items at the same time. So avoid this as much as you can. If a book you're using puts two similar ones together, delay learning one until later.
2. Focus on distinguishing characteristics and use memory aids to distinguish them.
If you notice two characters are very similar, learn one first. when you learn the other, pay attention to what distinguishes them visually, and try to make an association with the distinguishing part. For example, 犬 means dog and 太 means great. So imagine the extra stroke in 犬 is the head of the dog, and you'll be able to remember the difference.
3. Write the characters as you learn them.
Don't just learn the characters visually; learn to write them too. Adding tactile information to your learning will aid your memory.
4. Learn radicals and pronunciation components well.
Try to learn the meanings of radicals and the pronunciations of characters that are used as the pronunciation component of a lot of characters. If you don't mix up the parts, you'll have less of a problem mixing up the whole. I don't know Japanese Kanji, but I'll give an example from Korean Hanja. If you know 古 is pronounced 'go', you won't mix up 若 (yak) and 苦 (go); it will be easily remembered that the second one is pronounced go, and the first one is not.