The Kaf Hahaim (581:18) teaches that from the beginning of the month of Ellul, when one writes a letter to a friend he should preface the letter by wishing his friend a good year, and that he should be written and signed [in the book of life] leshana tova tichatev vetehatem. Similarly, some have the custom to conclude a letter with the letters kaf, vav, het and tet (ketiva vehatima tova). It would be proper to add this to ones standard signature on emails and text messages.
In addition, there is a custom during the month of Ellul to check tefillin and mezuzot, even though it is only necessary to check ones mezuzot every seven years. A way to remember this is to check the mezuzot on a leap year, during Adar, which will always be twice during seven years.
Finally, there is a custom, beginning from the 25th of Ellul, which commemorates the day upon which God began to create the world (i.e. seven days before Rosh Hashana), to strive to gain extra merits; some even say the Shehehiyanu blessing each of those days on a new fruit.
