Before pregnancy, make sure you are eating balanced diet and taking enough of the followings:
1) Folic acid
2) Iron
3) Calcium & Vitamin D

I would say the most important thing for planning a pregnancy is Folic acid. Deficiency of folic acid will significantly increase the risk of NTDs (neural tube defects). NTD occurs very early in pregnancy and neural tube closes at 28 days gestation. North America fortified flour, cereals with folic acid to increase folic acid level in common food, this ntrition policy is based on many studies.

Prior to pregnancy, the folic acid intake should be 400ug/day. During pregnancy, should be 600ug/day. Most women in North American are not meeting the requirements.If using birth controll pills, smoking, you might have low level of folic acid. Therefore, it's recommended to take prenatal multi-vitamins to ensure the folic acid intake.

About iron, rarely do women enter pregnancy with iron stores sufficient to cover all the needs. The daily intake is 13mg/d for the first 3 months, 18mg/d for the next 3 months, and 23mg/d for the last 3 months.

Calcium intake should be 1200-1500mg/d and vitamin D 200IU/d for pregnancy women. If you are drinking milk, that's very good. If you are concerning about not getting enough calcium. You can try the "Vicactive" calcium supplement. It's like a piece of chocolate, each piece contains 500mg calcium and 100IU vitamin D.