This has probably been said before, but it may be worth saying it in the IGI part of the forum:

Let's suppose you have found your ancestor's christening in the online IGI, as an "extracted record"--that is, a transcription of a Parish Register. (There will be a "Batch Number" below, probably starting with the letter C for christenings.) Now you can look for christenings of other children of the same surname in the same parish. Click on the Batch number, and type in just the surname. If there are too many results, you can add a date range, or add father's forename, or both parents' forenames.

With the parents' names and the christening dates of their children--the ones in that parish, anyway--you'll have an idea of when the parents married, and can search the IGI for their marriage.

If there doesn't seem to be a marriage in the right time or place, it is worth checking FreeReg www.freereg.org.uk.

So far, the Parish Registers for only a small percentage of English and Welsh parishes have been entered in the FreeReg database. You can run down the menu and see if your parish has been done, and if so, search for the marriage entry there.

FreeReg also has burials, so you may be able to find deaths for those elusive ancestors.

Another advantage is that, unlike the IGI (which is limited to only certain fields of information), a FreeReg entry will give any extra information that was in the original Register. You may find occupation, age, home parish, marital status, marriage witnesses, or odd tidbits like "died of the smallpox" in FreeReg entries.

Do remember to read the info pages for FreeReg. One important point is that the search feature looks for exactly what you type in as a forename. If you're looking for a Register entry for Elizabeth, for example, it won't find an entry that was originally written as Eliz or Elizth.