Apart from the standard material, I have found very valuable the WWI Service Records - many of these were destroyed in a bombing raids in World War Two, but I have been lucky enough to find pages on my great uncle George Danson (1897-1916) and also my husband's great uncle Frederick Donaldson (1894-1916). The records include personal descriptions, next of kin, signatures on enlistment, medical history and notification of death. Ironically both men, from different sides of the country. died on the same day at the Battle of the Somme - perhaps not surprising given the huge loss of life.
I had the basic subscription, but recently upgraded this, so I could access newer records that have come online e.g. British Post Office Appointments I found my two great uncles, but to be honest the record gives no more than their name and date of appointment. My next task is to look at the Railway Employment Records for more of my family.
The New York Passenger Lists 1820-1957 provided the information on my American connection - Alice Mason, nee Rawcliffe, who travelled with six children under 11 years old and two pieces of baggage. I took out the pay-as-you-view option to http://www.ancestry.com/ and was delighted to find her family (with five more children!) in American census returns for New York and New Jersey.