I always take them daily in first trimester, and I had good intentions to take them daily again in second trimester (I slacked off last pregnancy and didn't take them much past 20 weeks) but ended up slacking off again around 20ish weeks, and taking them sporadically when I remembered about them, and then starting up again daily at 28 weeks. Exactly what I did last pregnancy too lol. Now I take RL extract, RLT, and EPO and I remember my vitamins in amongst my daily schedule of pills and teas because I've worked out a time to take each ill that maximises absorption (with food and at different times etc, and apparently taking RL extract or tea can inhibit the absorption of other vitamins if you take things together, so the naturopath said).
Maybe it's because we tend to feel ebtter so we don't think about the vitamins. I know if I feel really crappy I always remember to take them inthe hopes that they'll make me feel better.