Rambam[1] writes that while it is never too late to do teshuva, optimally one should repent while at the same age and stage of life. Optimally the only difference between past cheit and present compliance with mitzvos Hashem should be the penitent's teshuva. Rambam adds that Shlomo Hamelech was speaking of this timing when he said, "and remember your Creator in the days of your youth".
Rambam hitherto had been discoursing about teshuva; he quotes proof text from Megilas Koheles where Shlomo Hamelech speaks about remembering Hashem. A fundamental, all important equation emerges. Teshuva means remembering Hashem.
The background to this equation lies in an understanding of cheit. Consider the following mashal. Even someone whose temptation to cheat on an examination is so strong as to potentially overpower his scruples will not cheat when the proctor is watching. Cheaters engage in their dishonesty when they feel that their actions are not being observed.
The analogue is clear. The yetzer hara manifests itself in different ways, pushing us in different directions. The temptation to speak or listen to lashon hara differs from the yetzer hara for inappropriate physical pleasure. There is a common denominator, however. Every yetzer hara, every manifestation of the yetzer hara, would be inhibited if only we were cognizant of being in the presence of Hakadosh Baruch Hu, who is always watching us. Cheit involves forgetting Hakadosh Baruch Hu[2], rachmana litzlan, thus teshuva means remembering Hashem[3].
As we seek, b'siyata dismmaya, to take advantage of the aseres yemei teshuva, we must, through persistent and consistent reflection, work to increase our awareness of Hakadosh Baruch Hu. We must constantly remind ourselves of the ultimate truth of Hashem's omnipresence, ultimately internalizing this fundamental awareness. We must not only confess our particular sins, but also more generally forgetting Hashem which allowed for those sins. As we recite the al cheits we must be careful to stress the l'fanecha, in Your presence. May we all be zoche to teshuva shleima mei'ahava, a kisva vachasima tova, shnas chayim v'shalom.
[1] Hilchos Teshuva 2:1
[2] See Rabbeinu Yona, Sha'arei Teshuva Shaar I, ikar 6.
[3] Besides Hilchos Teshuva 2:1, see also 3:4.