Bosses expect that employees will quit sooner or later. So just the fact of quitting will not make them think less of you. What will make them think less of you is how you behave during your notice period and what things break after you were gone that are a result of something you did or did not do (or let them know about).
I think one of the most critical things you can do is to not leave them with a mess to clean up. Think about what you would want available to you on the first day of a new job about your duties, where things are, etc. and create a transition document that lets your boss know everything critical that will need to be passed on to others. Let him know the state of all current projects - whether they are complete or how far from completion they are, where any files that will be need to complete the project are, any admin passwords you might have (that will remind him to reset them).
I prepare this document and give it to my boss at the same time I give him my resignation letter typically and then we discuss, who to transition projects to and what I can get completed before I leave. If you commit to finishing something before you leave, then make sure to honor your commitment.
