A bit of a wobble in spelling can be useful
Let me give you an example, my mother's family name is HEARD, but the family originally came from Ireland, and that means there are several different spellings, HURD, HERD in particular. Apparently the Irish often did this type of thing, I don't know the reason why.
Another trap is that records are transcribed, so the person reading the record can easily make a mistake. As a transcribe myself I know how easily that can happen. Lets take KERNOS, I recently found records under KIRNOS. This was possibly due to the immigration officer and a language barrier, or perhaps turning Ukrainian into English! I have also searched under KIRNIS and KERNIS.
It seems that often the vowels get mixed up, so try substituting some in names when you are searching, and see what you can find.
The other thing to watch out for is letters easily misread, N or M, T or P (particularly in older script), P and R, R and N - and the list goes on. G is a real problem, it can become C, O, D, B or E! Try substituting some of these in the name you are searching, you never know what you will find.
Silent letters are another trap, CLARK or CLARKE for instance, or CRISP or CHRISP. Silent E and H are common, so try adding them in or deleting them.
The letter S seems to be very problematic, and you should try adding or removing it, for instance make MARKS into MARK in your search.
First names can often be shortened, I have found Elizabeth can be Liz, Eliz, Lizzy, Eliza, Elisa, and I am sure there are more. It can often be useful to use a wildcard here, and search for SURNAME, E*. This gives you more 'hits' usually, but can just find that person you need. Complicating things even more are nicknames, I found a very large list of nicknames here : http://familysearch.org/learn/wiki/en/Traditional_Nicknames_in_Old_Documents_-_A_Wiki_List
that you may find useful.
The old way of writing a name can be different as well, I have found WILLIAM written as Wm in several old English records. There is a list of old abbreviations that you might find useful on reootsweb here : http://freereg.rootsweb.com/howto/realnames.htm
If you have scotish or Irish names in your family history search with all the variants, MAC, MC, MCc, with the O' and without.
All in all, my advice is, when you cannot find a record you think should be there, try some name variations and see what you find, and wobble a little!