Lately i've been seeing too many people who stick with one build and just play that instead of playing their enemies. I'll see the standard builds of swiftness, infinity edge, cleaver, phantom, maybe a veil. But the other team ends up having 3 champions with 3-4k health and they end up doing no damage.
You need to build to you enemies. You see 3 tanks? rush the razor first. not only will it help you push your lane and farm, it building into a bloodrazor will help late game. Because you are looking to go bloodrazor, attack speed is more important for the passive proc than pure damage. In this case a zeal would be good, along with a last whisper. Even another bloodrazor would be fantastic with the passvie staking.
If they have lots of dps (trist, teemo, twitch, ashe) auto attackers getting the ninja tabi boots are the better choice. A dancer would be a decent choice because of the crit chance and dodge. But you should focus more on pure dmg to take large chunks of heal. Infinity Edge, Cleaver, and a veil would be a good build strat. Getting a catalyst first to give you mana/health regen in the early portions of the game actually outperforms a Phil stone and it builds into the veil.
If you are against a team with heals, play conservative. Don't worry if you lose a tower or three. If they have 1 or 2 direct healers (teric, soraka) and you don't it means they are sacrificing late game dps for survivability. If you and your team can survive long enough to reach late late game, you should have more pure damage than them and be able to overtake them. Again in this situation you would want more dps and focus either their main dps or healers. Many times a dps champion won't get survivability because of the healer on the team. Makes them squishy and will drop quickly.
As far as boots go, Swiftness one ashe is the wost choice. Mercury will allow you to start attacking faster which means they die faster and adds MR(AWESOME!). Tabi adds survivability against dps champions. Berserker aren't as good, but the 25% AS has good synergy with the first build.
Again, you should never go into a game knowing what you will buy in what order. Always know what items they are getting so you can tailor your build to counter them.