Robitussin (or rather the $2 target brand) did help me this month as well. I have also heard that grapefruit juice works for that so I tried it too. Not sure exactly which helped though.
I pulled this info off babyhopes.com. It should answer most of your questions - good luck!
Robitussin is a popular brand of cough syrup and it might be surprising to consider its relation with fertility. But the fact is that this expectorant is used to loosen and thin mucus of a different kind too, called cervical mucus, in women. Robitussin is only a specific brand and any expectorant that contains guaifenesin as the only active ingredient can be used in its place. Generic versions are most likely available as well. Be sure to carefully inspect the ingredients list to make sure the ONLY active ingredient as guaifenesin. Any other ingredients can diminish the effect you are looking for.
The recommended dose for improving cervical mucus is two teaspoons (200 mg) taken orally three times per day. Recommended dosage is two teaspoons (200 mg) taken orally three times per day. If mucus still appears thick, you can take as the maximum dosage as listed on the label of the cough medicine. Water intake should be increased to encourage cervical mucus production and a full glass should take with each dose of guaifenesin.
Most doctors suggest taking Robitussin five days before and including the day of ovulation for a total of 6 days during your cycle.This helps provide the optimal environment to help the sperm survive and get to where they need to go.If you take Clomid, waiting until the day after the last Clomid pill is taken before starting Robitussin is suggested.