In dragon attacks, the biggest threats are air defenses. In a way, TH8 and below are the bases where Dragons shine best; there are 3 or less ADs (Air Defenses) at those TH levels.
There are a couple of staple effective ways to deal with ADs:
Using lightning spells: 3 lightning spells can usually take out an AD. 3 lightning spells maxed at any TH level are sure to destroy an AD maxed out for the same TH level. For a TH7, if you have lv4 lightning spells (390 dmg*3 = 1170) and the AD are lv 5 (1000 HP) you can easily take out one of the ADs.
Using support CC: If you have access to high level troops from clanmates, usually the best troops for this attack are hogs and balloons. 5 maxed out hogs or balloons can help destroy one of the ADs and at TH7, the defenses will definitely take a while to take those troops out.
The strategy is to destroy 1 AD from the method above and the other with the Dragons. If you bring lightning spells, it would be better to destroy 1 AD early before the main attack and then send all the Dragons so they destroy the remaining AD as fast as possible.
If you bring Balloons to destroy an AD, then you bring Dragons on one side and send the Balloons when or slightly before the AD on the other side of the Dragons start targeting your Dragons. The goal here is to prevent the AD from targeting your Balloons while they come in and maxed Balloons will come with a big bang and will help your Dragons even after destroying the AD.
Hogs can be sent earlier since they are not targeted by the AD, but you have to make sure the CC is empty first, or rather, no defending CC can attack them while they do their thing. You can send your Barbarian King in front of them if there are cannons or Archer Towers between the AD and where you are sending the Hogs, so that he draws fire on him thus protecting those Hogs. Be more aware or possible double giant bombs when you send those hogs, because that can kill all your hogs before they reach the AD and hence cause you the last star.
Notice that I haven't mentioned the defending CC troops that much; and that's because dragons can usually take out the defending CC troops with ease.
The same strategy can be used at TH8, where the dragons will have to potentially deal with 2 ADs rather than 1, but if the ADs are close together, maxed Balloons under rage will be able to take them out relatively quickly and that leaves the Dragon freer to roam the skies.
EDIT: Per late 2015 update, you can use 2 lightning spells and 1 quake spell to destroy an AD instead, which leaves you an additional spell for your troops and usually the best to use is a rage to speed up those big bad dragons :)