The rule of thumb is to give yourself at least 6 weeks to plan your packing strategy, but life doesn't always happen by the rules. Often, you'll find that you have less than a week left and will start freaking out about how to pack your whole life into a bunch of boxes.
A common sense strategy is to begin boxing things you don't plan to take with you. These you can "gift" to "friends" as you say goodbye: Thanks for all the Monday night football parties, Tim. Here's a semi-used crock pot for next year's chili cook-off.
You can also donate stuff you don't need to a local non-profit before you leave. Take stock. Do you really want to move that futon, or is it time to upgrade to a mattress and box springs?
Pack non-essentials and items you can do without first. For example, it's July. Winter jackets and ski clothes get packed before anything else. You won't need them for a while.
Label your boxes. For essential items like towels, soap and shampoo, add "Load last, unload first" so you have access to the things you need when you get to your new digs. Like the screwdriver to assemble your bed when you arrive.
Take care of valuables. If you've got important paper records or jewelry, either keep them with you or send them using a secure courier company. If it's priceless, keep it with you. No use fretting about the safety of your signed and framed photo of Fabio while you navigate the move and organize the rest of your life.