my usual staregy for base defence (at least early game) is to first set up a choke point to slow alien egress, in an early game base I usually have about 14 soldiers (10 men for main team, 4 to replace wounded soldiers) and usually 1 or 2 rocket tanks.
first turn I move my guys into position near the choke point, usually with a couple of guys held back for sniping from the far end of the base while the rest move in close.
of my team 5 are the heavy weapon team (1 rocket launcher with a heavy rocket and as many light or heavy rockets he can carry, 2 auto cannons with HE ammo, and 2 heavy cannons with HE ammo, all with pistols as a backup) the rest are the rifle team armed with rifles, grenades and smoke grenades. (med kits, motion trackers and stun rods are carried by the rifle team depending on availability and their strength)
when it comes to the battle, I use my snipers to pick of exposed enemy, then leap frog my team into the access lift using available cover, smoke grenades and liberal application of explosives to keep the enemy pinned. my tank is usually the first into the access lift with troops backing it up, then again the first into the hanger.
it's then a case of clearing the hanger one by one, again the liberal use of explosives especially rockets is great here as there is little to damage, the explosives clear a larger area and create smoke which hides your movements from the enemy. where possible use the tank as a spotter while rifle teams snipe the aliens
this tactic keeps up through mid game when you have armour, and heavier weapons such as plasma rifles. with small launchers replacing heavy cannons.
by the late game you can use psionics to take control of enemy units, then chain it to control as many as possible before using them to kill any uncontrolled unit or each other (nothing like mind controling a dozen mutons to line up against a wall while you shoot them all firing squad style
also as for how often base defense mission happen, this depends on several factors, if you're doing really well, you can expect to have the aliens begin searchhing for you base from the get go, while if you're doing bad they won't bother, then you have the fact that if you shoot down scouts and such on a retaliation mission before they find your base then they'll never find it to launch an attack. in my last game I was getting 2-3 alien retaliation missions a month but the scouts were being shot down far from the base so there was only one base attack (which was early in the game when I only had radar and didn't spot the scouts earlier.
