One word: Don't. Unless you have a real deficiency, it is much better to tweak your diet to get all you need. Studies have shown that vitamins from pills don't work on the body like those from food. Simply put, our bodies cannot really deal with synthetic nutrients. More on this here and here. In fact there is evidence that people who take synthetic vitamins live shorter.
If you do want to take vitamins, look for natural-based instead of synthetic ones (hint: if the ingredient list says "vitamin A", it's synthetic. If the list mentions extracts from herbs, vegetables etc, it's natural). When I was pregnant I was prescribed the ubiquitous iron pills. My midwife told me to throw them away and get what they call here Eisenblutsaft (iron-blood-juice) -- a syrup made of iron-rich plants.
A varied healthy diet is the best way to make sure that you are getting everything. Variety is key here, especially as veggies now contain way less nutrients than they used to. There are plenty on online charts to guide you that show which foods contain which vitamins and minerals.
Personally, I try to love to eat the same veggies all the time, but I make efforts to vary my food: I eat different kinds of grains (quinoa, amaranth, wild rice, kamut, etc). I blend the green leafy parts of many root vegetables (like carrots) in my green smoothies. Sometimes I use wild weeds too (like nettles). I don't digest nuts well, so I put lots of different seeds in my salads (flax, sesame, pumpkin and hemp are my favourite). I also add seaweed sometimes. I also use superfoods -- raw cocoa, maca and bee pollen. And I sprout. Sprouts are probably the most nutritious and cheap thing you can find. I also eat berries and exotic fruits from time to time.
I don't go crazy though --- I don't buy any expensive superfoods (ok, apart from raw cocoa --- the pollen and maca are bought for cheap by my friend from Peru). I add just one or two things at a time into my meal.
If you are too busy, you could make throw some seeds / superfood into a jar (like this lady) and then just add it to whatever you are eating.
