Classic Battle Line
The classic formation in naval combat is the battle line. This formation favors ships with strong, long range broadsides like the Assault Frigate and MC-80. With options for 'kiting', the standard battle line allows you some control over range as you are maneuvering. You can also force your opponent in to flying directly into your best firepower while constantly threatening your line moving around their flank.
One way that the classic battle line restricts you is in order of activation. For ease of maneuvering it often makes sense to activate the lead ship first. This can make you predictable and easier to counter. I often fly my Assault Frigates at speed 2, but make sure that I keep a Navigate token available. This is for two important reasons. First, it is much easier to stay in formation at speed 2 than speed 3. This helps maximize my concentrated firepower. Second, with that Navigate token I can easily alter my speed in either direction. At speed 3 it is possible, with practice, to jump the second ship past the first. This will often surprise your opponent and catch them off balance.
You can steer your line a couple of ways. The most common is to try to fly around one of your opponent's flanks. This brings the full power of your battle line against one flank of your opponents fleet. You can also fly along your deployment edge, essentially strafing your opponents fleet. For this reason, I usually try to start a game with my line at about a 45 degree angle. I can then adjust my heading based on the flow of the game.
The classic line has two real weak spots, directly ahead and behind. In both of these locations, there is very little firepower available. If someone can get directly in front of your line, they can also cause a chain reaction of collisions. You can avoid getting cut off by being flexible in the heading of your line. The rear of the line is also a weak spot as the lead ships will usually have trouble getting firepower there. This is usually where support ships can help. You can task a frigate with following a ways behind your line to engage any ship that tries to sneak in behind you.
Another great formation, that I think favors the Imperials, is the Echelon. The Echelon basically involves flying your fleet as a diagonal line.
The first tactic every player learns against star destroyers is to quickly fly past their front arc. With an Echelon formation, you will have the rest of your ships waiting off one of those flanks. So, I know what you're thinking. What about the other flank? Well, that's where smart deployment comes in. Your lead ship in an Ecehelon needs to be deployed farther outside than your opponents main forces. (I really wouldn't be too worried if they sneak a corvette out there) Essentially, you are hoping to 'refuse' one of your flanks and make the other easier to cover. The ISD will make a great lead ship in Echelon, as it will have the speed to turn the corner and bring the formation around on the opposing fleet.
In games where I have used an Imperial Echelon I have been able to herd my opponent into my best firepower. It's a great way to protect the flanks of the VSD. Tractor beams will also be a boon to this formation.
Echelon formations will take some practice in turning in a coordinated fashion, but you can really bring a lot of firepower to bear.
The line abreast almost resembles a line of soldiers slowly advancing across a field. It creates an area of maximum overlapping firing arcs that spell death for any enemy ship foolish enough to fly in front of you.
This approach is currently favored by lines of Nebulon-B frigates and I've seen it do quite well at tournaments. The line abreast really allows you to pick an area or ship and apply tremendous firepower. The closer the ships are in the line, the better the concentration of firepower. Line abreast fleets often move forward slowly, attempting to maintain range as long as possible.
The obvious weakness of the line abreast is being flanked much like a Star Destroyer can be. If you are going to use this formation, it is important planning out when and how to turn once you know which flank your opponent will be attacking. At Nationals, I flew my battle line towards the flank of a line abreast opponent and he was a turn too late in turning, which allowed me to get his flank and take him apart.
Somewhere between the Echelon and the Line Abreast is the Wedge.
The Wedge creates a formation that is very difficult to flank. It allows you to place supporting firepower in your weak spots. It requires more ships to really work well than other formations, so, will probably be more common in 400 point games.
In a Wedge, the weak point is really the lead ship. Rather than emphasizing firepower, this ship would likely want to emphasize defensive upgrades and Engineering commands.
Much like a Line Abreast formation, with a Wedge you would want to practicing turning your ships towards one flank or the other depending on where your opponent concentrates his power. If you don't the ship on the far corner of the Wedge risks be blocked out of contributing its firepower to your fleet.
No matter what fleet formation works best for you, I really encourage you to think about it and practice a formation each time you build a list. Don't spend hours agonizing over which is the best turbolaser upgrade to use without even considering the best arrangement and spacing of the ships you intend to use.