Many people want a particular post to stay always at the top position on your homepage. Actually, Blogger arranges the posts order according to their date & time. So, you can also change your post time to keep it at the top as long as you wish. There are two scenarios explained below.
If you're writing a post and haven't published yet
Just click the Post Options link at the bottom of the post editor and change the date of the post to some future date like 2020. The format of the date is Month/Day/Year like when I'm writing this post, the date is 7/15/09, so I can change 09 to some future year.
If you've already written and published the post
Then go to Edit Posts and then find that post. Once found, click Edit link and post will again open in the Post Editor where you can change it's date.
It's a pretty simple tutorial, stay in touch for more tips and tricks, see ya!