It depends on how much of a rush you are in. We moved 500 miles a little over a year ago.
We started by purging, giving furniture back to family that they had given us (my MIL loves to pass things along to us, which is fine, but not when it clutters up our house!), etc.
Pack up things right away that you won't need for a while. If you have winter where you live, maybe start packing up your summer clothes, or paring them down, for example. Pack up things you only use a few times a year, or could do without.
In terms of moving/purging, realize that moving is one of the few times in your life where you will touch everything you own at least once or twice, kwim? So take advantage and purge now, unless you are in a tremendous hurry to sell ASAP. Purge when you unpack as well...you can always get rid of more.
We started boxing things up waaaay in advance, and it was still tough. We did a sticker system. All things that were long term storage (could me moved immediately to attic, etc. in new house) were boxed as early as possible and given round stickers on all sides of the box that were red (can buy these at Staples, etc.). Red became "low priority" to unpack, or didn't have to be unpacked at all (baby items at the time, since we weren't yet expecting #2, seasonal decorations, etc.). We gave some things "yellow" dots which were for items that didn't need to be unpacked immediately upon arriving in our new home, but would be unpacked within a month or so (a lot of DS's books since he has tons and tons, craft supplies, etc.). Green dots were given to boxes filled with things that needed to be unpacked ASAP when we arrived. LABEL, LABEL, LABEL everything, on all sides of the box. Seriously. Specifics whenever possible. Some people even do a box inventory where they keep a notebook with a running list of what is in each box, and number all of the boxes. That way if you really need something when you arrive, you'll know where it is. Some people take pictures of certain valuables, etc. as they box them so that if something should happen in transit, you'll have it for insurance purposes. www.organizedhome.com
has some great moving tips IMO.
We pared down rooms, etc. as much as possible when showing our house, and I think it really, really helped the house show well. It was also way easier to keep up with cleaning and last minute showings that way. We even did things like buy extra burner covers, etc. so we could just swap them out before a showing and the kitchen would look extra clean. We bought new shower curtains and pretty towels for both bathrooms, which also really spruced things up and they can be taken with you at least. We bought new throw pillows for our master bed. Small purchases, and I think we spent under $100 but those things can help "stage" your house a little more nicely for the price. Oh, and I really pared down our walk in closet and hung clothes organized by type and color. Way more anal than I normamlly have time for, but it made a big difference in appearance, IMO. If you minimize how many extras are in your linen closets, etc. it makes them look more spacious. If it looks like they are bursting at the seams, buyers wonder if there's enough storage space, kwim?
When it is time for the actual move, keep a box of necessities for each of you, or a big box plus a few suitcases that you keep with you in your own car. Toothbrushes, toothpaste, hair brush, medications, a few rolls of toilet paper and paper towels, a bucket of cleaning products, 2 changes of clothes for each family member, pet food and bowls, some bathroom towels, etc. That way you'll have the necessities available immediately. You can throw everyone's in together or do a kit for each family member.
ETA: we looked into PODS, but at least when/where we were looking, the rental price was quite expensive IMO. It might be worth it if you really don't have storage space, but we just moved as much as we could to our crawl space and basement in our old house, or got rid of things. I wouldn't want to have to pay the PODS rental fee in case my house didn't sell quickly. Just be sure to price out other storage space options if you can, to compare. There may be something cheaper, or you may decide the price of the PODS is worthwhile.