Recently I read a meta-analysis of what works and what doesn't for a common cold. You can find it here.

Some highlights from that article:

Vitamin C
Prevention
Generally does nothing if taken as a prophylactic, but...
"prophylactic vitamin C (when given to marathon runners, skiers, or soldiers in subarctic climates) reduced the incidence of the common cold by 50%. This appears to suggest that, when taken during periods of stress when the body's immune system may be weakened, vitamin C may help to prevent the onset of the common cold." and
"In addition, among individuals who experience a cold when taking vitamin C preventively, the duration of the cold may be reduced by 8%; the impact on the severity of the cold may only be slight, however"
Take home message - it probably makes sense to take vitamin C during the times of stress to prevent cold. If you do take vit C chronically your colds will be shorter by a whopping 8%!

Treatment
"one study did find that a single 8-g dose of vitamin C, when taken at the initial onset of symptoms, reduced the duration of the cold to 2.86 days, compared with 3.52 days with placebo. Vitamin C treatment does not appear to reduce the severity of the common cold, despite one study that found a slight benefit to its use"
Take home message - 8 grams of vitamin C in any form (even the "good" Ester-C kind) will give you epic diarrhea, but it may also cut the duration of your cold!

Echinacea
Treatment and prevention
"echinacea versus placebo for prevention, no statistically significant difference was found in terms of the number of colds, their severity, or duration"
"Two studies evaluated the efficacy of echinacea versus placebo in individuals who developed a complete cold on the duration of the cold. Whereas one study showed statistical significance, the other did not."
Mixed results in both treatment and prevention - it might or might not work

Zinc Gluconate
Lozenges
"The author of this review concluded that fair evidence suggests that zinc gluconate lozenges reduce the severity and duration of the cold when taking at least 13.3 mg of elemental zinc every 2 hours while awake starting within 24 to 48 hours of symptom onset"
Intranasal
"zincum gluconicum nasal gel spray shortened cold duration and reduced the severity of symptoms if initiated within 24 to 48 hours of symptom onset."

Although some studies have shown zinc lozenges to not be effective - there is significant evidence that they do work. Nasal gel seems to work the best

There was an article in the newspaper today revealing that chicken soup does indeed help fight off the common cold, or at least the symptoms.

I can't explain why but I've noticed vitamin supplements work pretty well for me. I wasn't taking multivitamins for some years but started again a few months ago and there has definitely been some improvement. I've even managed to stay healthy when everyone around me got sick and I'm pretty sure at least part of it is due to the vitamin supplements. Working out more intensely again probably helps, too.

A hand bell is also an essential piece of kit to help deal with a cold.

http://uk.youtube.com/watch?v=rXLHWmjA5IE

In another post, I mentioned Airborne (high vitamin dose mixed in water) and Zicam (zinc) spray (not the lozenges)ended my cold 3 days before my wife, who caught it at the same time and did not treat with any nostrum.

I also mentioned I am a big believer in the power of positive thinking- I truly believe that Zicam and Airborne are going to work, while my wife is resigned to riding it out.

I think that's why placebo studies are so prevalent (and so erratic, sometimes)- folks think they're getting a cure, and their belief (or non-belief) affects efficacy.

I remember what my family doctor used to say:

"If you have a cold, you should get plenty of rest, eat healthy foods and drink plenty of fluids. You'll feel better in about 7 days. You also could do the same things and take plenty of expensive over-the-counter medications. You'll also feel better in 7 days."

Homemade chicken soup!!

When I make it, I make about 5 gallons and freeze a bunch. Dont freeze with the noodles in it, they nearly disintegrate.

Big Daddy, I believe it helps, thus, it does. The settlement is about lawyers, naturally, and package labels.

I've had great success with my formula:

Get plenty of rest and drink plenty of fluids.

Although I do take a daily multi-vitamin and "prophylactic" Vitamin C as well, for some reason I am less sick and less often than my athletic friends.