Off the top off my head, the following comes to mind:
1. Ward by the clock. Learn to recognize the times when wards expire and are available for purchase.
2. Adjust your ward positions accordingly. As you take more towers, you can afford more aggressive wards. The idea is to control as much of the map as possible, denying your enemy space to farm. Creative warding also helps avoid ward wars, which are generally a waste of time and gold. In the event of an enemy player who's particularly aggressive at warding, it's a good idea to ward under the cover of smoke.
3. Dual supports are the best because they can share the cost of consumables. The more item-dependent support (4th position) should pick up/upgrade the courier as well as get smoke. This allows him to move on to his core items sooner. The less item-dependent support (5th position) can spend gold on observers/sentries/dust.
4. If you're forced to support alone, focus on observers first. Don't bother dewarding unless it's absolutely necessary. A solo support doesn't have the income to get into a ward war. Having more carries on your team means you only need to provide the necessary vision so they can farm up their items.
5. Gank/pull consistently. Being missing from the map means the enemy players have to play it safe, impeding their farm. Time your ganks every 2 minutes, to coincide with rune spawns. A favorable rune can be the difference between a successful and an unsuccessful gank. Unsuccessful ganks are a waste of exp and gold, so decide beforehand whether a rotation will be worth it.
6. Pick up items that benefit the team. Meka/Pipe/FS/Arc/Vlads/Urn are all excellent items. So are Necro/Medallion/Veil. The former are defensive sustain items, while the latter are offensive. Adjust your item choice depending on what your hero/team needs.
7. Communicate with your team regarding item choices. Double Mek/Pipe etc. on a single team is a waste of gold. Multiple Arcs/Necros can be useful, depending on your lineup.
8. If you're playing a squishy support against high-DPS heroes (Naix/Alc/Viper etc.), Ghost/Eul is your friend.
9. Positioning is important. Don't take bad wars. Solo TP-ing into a team of 5 enemies pushing your tower is suicide. So is fighting uphill without vision, or around massive blind spots.
10. Avoid TP-ing to a lane from base. Keep your scroll so that you can TP to help an ally in case of a tower dive.
I disagree. In fact, most players recommend going CM if you're teamed with 4 carries, as she has very good early game impact if played well. Good harass potential, aura benefits the entire team, and her nukes deal a shit ton of damage at levels 1-5.