One of the most effective tactics that I employ is a flanking maneuver. The most effective way to do this is with fast hard hitting units. With my sisters I use an 8 girl Seraphim squad supported by a jump packing Canoness, the rest of the army acts as the hard point from them to swing from. Your opponent will usually try to dedicate some unit to take out your flankers, but they aren't prepared to deal with the speed and fire power that the unit (or units) can put out. Many a game they will put dedicated assaulters or shooters protecting their flank only to see the unit ripped apart and spit out and then your flankers smashes into their expensive command squad or most devastating vehicle. They will squeal as they see all of the hits smash through their guys and wine as units run off the board or get totally annihilated. So when your opponent winces when you put a flanker wide you know that they are scared of it.
The only way that you can do this effectively is if you set a hard core the helps the flankers draw less attention, then when it is too late you close into their lines and wreak great piles of havoc. The core or the swing point should usually be your main army (troops and other slower moving units). This should always be big and flashy so that they worry about it. You should always worry about those really amazing tacticians that see the flank and devastate it. In those situations your flankers are now more of a distraction than anything. The fast and more devastating the squad the more you might have to worry that your tactic will spread within your gaming group. This tactics works best when you take your opponent by surprise.
A good way to take your opponent's eyes of the flankers is to deploy the rest of your army way in the other corner of the board. This, while leaving flankers exposed, allows them to get to their targets with little to no damage but means they have to travel longer to get to the main battle. This can create interesting plays where you basically have 2 different games going on. If you do it this way keep your opponents army focused on a big tank or killer squad so that they don't try to support their flank. Keep them off balance. If one of you breaks towards the other side one of the games will be lost. Hopefully you win the bigger of the 2.
If your opponent employs this tactic against you give him a weak squad that is in good cover and well supported. So that he will go after it foolishly and expose his own flank so that you can push through the gap and play havoc upon his army. Your other option is to flank the opposite way strong siding his flankers, this tactic is very dangerous and you should pray to the dice gods that your flankers can survive long enough to finish their duty. If you do this you and your opponent have one strong side and one flank. If you know that your flankers can handle a large amount of fire or are very maneuverable then you should be fine. Always try to evaluate your opponents set. Never give him flankers that won't get to make up their points.
If your opponent is setting up in a straight line across the board look to see which units are going to be the easiest targets and will get your flankers into the best positions. The last thing that I would like to say is that don't drop to much into your flankers more then 300 pts is too much. You should always be wiling to give them to our opponent as a distraction. Happy Gaming!