There's some basic information that I think will help you to know for shifting properly.
How hard or easy a gear is to push is expressed in "gear inches". It is nominally the amount of distance your bike travels per rotation of your pedals/crank so the lower the gear inch number is, the easier it is to pedal. A previous poster looked up your bike and said it had a 34/50 on the front and a 12-28 cassette on the back.
This is how that calculates in gear inches:
34T Chainring : 32.8" / 35.3" / 38.3" / 41.7" / 45.9" / 51.0" / 57.4" / 65.6" / 70.6" / 76.5"
50T Chainring : 48.2" / 51.9" / 56.3" / 61.4" / 67.5" / 75.0" / 84.4" / 96.4" / 103.8" / 112.5"
This is not something you need to memorize and there will not be a quiz at the end of this post. The important thing to see here is that the gears between 50" and 75"each have a nearly duplicate gear in the bottom and top chainring. Here is why that is good to know:
Lets say you are cruising along on a flat stretch of road in the big chain ring in the front and 4 from the top in the back, you would be in a 61.4" gear. As you look down the road you see you have a significant hill coming up, you know you will eventually need one of your easiest gears to go over it. As soon as the going gets even the slightest bit harder you shift down to the small chainring in the front and simultaneously shift down 3 clicks in the back, now you are in a 57.4" gear. And for the rest of the uphill you will be in the chainring you want to be in and just click it up one cassette ring at a time as needed until you get to the top. Going down is the opposite. You've just gone over the top of the big hill, you've got a mile in front of you at a 8% grade, as soon as you pick up any real momentum, take it out of small chainring and move it to the big one because you know you'll soon be going 30 -35 and you're going to need that 112" gear.
This method, particularly used while going up hill will keep you from ever having to shift between the front chain rings while you're under load. Something you may have noticed you were warned against by several previous posters.
This has already been a long post but a couple more things:
1) the gearing you have on your bike is most likely a little stout for a 400± mile trip carrying gear. You probably want to get a bigger cassette or a triple on the front before your go. And if you want to look up your new gear inch ratio's, here is a link : http://sheldonbrown.com/gears/
2) I don't know how new your bike is and if it has the shifters in the brake levers, but if it doesn't you may want to update it or at least get bar end shifters. As a relative newbie it would be a good idea if you can keep your hands on the handlebars while you shift. I've been riding for a long time and the brake lever shifters are the only improvement in bicycles that I consider really important in the last 40 years.
Congratulations on your new lifestyle and keep at it, you are a inspiration!