tenses

In my writing, the most commonly recurring problem people see is I continually use different tenses. How can I fix this?

I have an elderly dog who tenses up & starts whining randomly? What's wrong with him? We adopted him & we aren't sure of his age. But when he tenses up sometimes he poops and pees where he is. Can anyone explain what might be wrong with him?

...form.? thanks in advance!

What do you think about switching tenses, just for one scene? I have a dream scene which would really sound much better in present tense, should I switch tenses? Or is it just weird? Also, can someone critique this: http://answers.yahoo.com/question/index?qid=20101124141405AAlXrXH