"All warfare is based on deception."
Surprise is hard to achieve in a game where all your forces are deployed in full sight of the opponent, and where they have full access to the same rules that you do. This doesn't mean that it cannot be achieved, and utilized, however.
There are several ways to generate surprise within your units. First, is transport surprise. Unless you're playing in a tournament where you're required to announce the contents of all transports, then by the rules in the rulebook, you don't have to tell your opponent anything about them. You can keep a transported unit off the board until such a time as they do something.
Naturally, this opens up several ways that you can bluff. Bluffing is a time-honoured tradition is most games, and there's no reason why it shouldn't be in wargames.
First is the questionable threat. Given a transport, pay as little as possible for the squad it transports. But, when it comes to using the transport, make moves that would indicate to your opponent that it is full of something far more dangerous than it actually is. They're bound to pay more attention to this transport than it warrants, making it an ideal sacrifice or bait unit.
Second, is the hidden weapon threat. Many vehicles allow you to fire a weapon out of the back of the transport. Depending on your army, and how you usually play it, your opponent may not expect to be facing flamer templates, or meltaguns that your unit is carrying. Flamers make for an exceptionally good surprise here, because if your opponent doesn't see a flamer, they may well not make any effort to spread their formations out.
Never underestimate the visual impact that a unit can have on the opponent. A weak unit can look extremely strong just by its size. A unit of conscripts may put 20 to 40 bodies on the table. You can keep the cost low by not giving them any upgrades, but the sheer number of models will still make your opponent worry about them, even though they probably cost under 200 points.
A weak character can be made to look extremely heroic and powerful with a good conversion and paintjob. A strong character can be hidden in mediocrity by painting him little different than your rank and file troopers.
If you regularly play against the same opponents, you can surprise them by breaking away from your standard equipment and weapon upgrades. While I don't advocate breaking the WYSIWYG rules, simple changes can be overlooked by an opponent who is just expecting you to do the same thing, game after game.