Hmm, it's fair to say that there are a few well-established strategies.
Stage 1, Point 1: Rush an uber pyro/demo out of the top-left exit straight into the hut, taking out the sentries. Rest of team follows and caps the point.
Stage 1, Point 2: Try and capture the point immediately after getting point 1, as the enemy team will be mostly dead and you can often get a quick cap. Otherwise, build an uber in the top-left passage, behind the grate; get an offensive engineer to build a sentry and teleporter there to defend the medics as they build it up. Then, rush down and cap the point.
Stage 2, Point 1: The enemy team will often rush an uber pyro into the offence here. The offensive medics should hold back and save their uber. Then, use soldiers and sticky-jumping demos to quickly cap the point.
Stage 2, Point 2: One of the hardest points to cap. Offence often builds up an uber in the right-hand tunnel, behind the grate, in an identical way to S1P2. Learn all the sneaky ways to jump up to the large building outside the point; there are barrels, crates and walls you can jump on to get the height advantage. If there are a lot of sentries on the final point, you often need two ubers; one uber demo to break the sentries, and one uber HW/soldier to push the defenders back.
Stage 3, Point 1: Demomen can defend this point very well by carpeting the route to the point with stickies, while engineers build around the corner. Offence should use stickyjumping demomen and spies to steal the point from under defence's nose. Once this is done, defence will start to get pushed back and the point can be secured.
Stage 3, Point 2: Quickly have a soldier/demo jump into the high passage from red spawn and take out any teleporter exits, to stop them flanking you. Also guard the low passage from the chasm to stop pyros/spies flanking your team. Then, slowly push up the winding top passage using medics, HWs, soldiers and maybe 1 offensive engineer. If you have a good sniper and the team can defend him, he can wreak havoc on the defending team. An uber will decide the point.
In general, remember that on offence you must kill one defender per ~18 seconds of play in order to overcome their respawn rate. If you're not killing one defender every ~18 seconds, your team may be behind in its momentum.