One of the most vivid ways in which the warning is conveyed is by blowing the shofar every weekday morning (in some places in the evening too – Shul. Ar., OC 581:1 and the Rema’s note).
The practice derives from Moses ascending the mountain for a second time on Rosh Chodesh Ellul and leaving instructions for the shofar to be blown in the Israelite camp in his absence to remind the people not to sin again.
In a classical passage Maimonides explains the shofar on Rosh HaShanah as a wake-up call – a thought that applies during Ellul too. If we leave the shofar to Rosh HaShanah we may be overburdening the day, and we need to get ourselves ready in advance.
The goal is summed up in the letters of Ellul – alef-lamed-vav-lamed, the initials of Ani l’dodi v’dodi li, “I am my Beloved’s and my Beloved is mine” (Shir HaShirim 6:3). Like an engaged couple who announce their love in the lead-up to their marriage, we should use Ellul to announce our love for God and His for us.