If I may pitch my two cents... Hello all.
In Japanese, love is as multifaceted as the language. There are different degrees to it and different ways of expressing it according to the situation.
"suki" is used both as like or love and the meaning has to be interpreted from the context. Therefore if one says:
"ramen ga suki"
It would be interpreted as meaning "I like ramen". But if one says:
"ramen ga dai suki", the use of the "dai" which means big compunds like into love, thus making the example above "I love ramen" in English.
When talking about people, suki can also be used to express filial love, special friendships, or childish love. So you could say "anata ga suki" to your mother, siblings, good friends, etc. When used towards boyfriends and girlfriends by adults, it has a childish and not too serious flavour to it.
Now "ai" is more laden with commitment and permanence. It is very rarely used in anything but the romantic sense. So, "anata wo aishiteiru" is usually used by adult lovers, husbands and wives, in declaring ones "serious" love to another, etc. However, when admitting feelings for someone, you would not use "ai" but rather something along "anata ga suki ni natteiru" (I'm beginning to love you).
"koi ni ochita" is more often used in a litterary sense, but when used in daily conversation, it is often used to talk about a third party falling in love as this is exactly what the expression means, "ochita" being the past tense of "ochiru", meaning to fall.
As for "koi wo suru", it usually means that you are flirting or active on the dating scene. For example, as a married man, if I said of myself that "koi wo shiteiru" I would be implying that I'm sleeping around or have one or several girlfriends. As a single person, it would mean that I'm dating, but not commited to anyone.
Does this clear things up some? I hope so. (I posted at the same time as Nangi made his post, so sorry if they overlap).