I just type them in MS Word and use an easily fileable layout.
Some commercial customers just pay on the spot from petty cash or a cheque on my giving them one of my "slips". Others want an invoice of which I produce about a dozen a month.
Viz e.g.:
Invoice no:- MG2005-09-04 = Year=2005 Month (09)= Sept 04 = 4th invoice of the month.
As the months and years go by they will automatically appear one above the other in Words file directory. So if I need to call one up I go into my documents/window cleaning stuff and then scroll down to my invoice. Every so often I store them to CD - perhaps every three months or so.
As file names they appear like this:- (eg)
MG2005-12-08 Wild Pursuits climbing centre Contact Eileen Overr Tel: 0123 678 456
MG2005-12-09 Pont Havren Water Authority Contact I.P. Daley Tel:0123 456 789
MG2006-01-01 ABC Hotel Group Contact Kurt Manners Tel:0123 097 654
MG2006-01-02 Wessex Maths College Contact Lois D. Nominater Tel:007-003=004
Most of them pay me without chasing by DD or a cheque lands on the mat. Luvverly!