Learning Romanian vocabulary is for most of us is time consuming, but there are different techniques available which may or may not speed the process up.
Before beginning to learn Romanian vocabulary, it is useful to divide the words you want to learn into two different categories, those you want to use (active vocabulary) and those you want to understand or recognise (passive vocabulary).
Everyone needs to know the word Bună dimineaţa (good morning), but not everyone needs to know the word poppy.
There are many different methods for increasing your vocabulary ranging from word association, memory palaces, mind maps and spaced repetition systems all of which are described in varying levels of detail all over the internet.
I'm only going to include the methods that I've actually found useful, as this site is as well a resource on Romanian, a personal journey of discovery.
My favourite and lo-tech method of learning words is by writing them on bits of paper and sticking them to objects. If you walked into my kitchen you would see the word bucătărie (kitchen) stuck to the door, and as you glanced at the fridge you would see the Romanian word frigider (fridge) and so on.
The reason this method is so effective at building a basic Romanian vocabulary is that you see the words associated with the relevant objects many times each day - at space intervals. In other words, it is a low-tech solution to a spaced repetition system, with the advantage that you can't get bored of using it.
This brings me to spaced repetition systems which help you to memorise information efficiently by testing you at gradually increasing but strictly controlled time intervals.
Spaced repetition systems work well, but I personally struggle to use them effectively as I find them incredibly boring to use, and prefer the low-tech label method where possible. If I do use them, I prefer to learn a word as part of a sentence.
I write words and sentences out by hand.
I know this is another lo-tech method instead of using memrise or a similar free internet resource. I'm not doing this because I'm a luddite but rather for me this is a more effective method of learning and retaining information.
If you haven't learnt a language before, then key point is too try different methods of learning and to see what works best for you.
I'm tried memrise and I've tried spaced repetition systems, and I've discovered that for me labelling objects and writing sentences out by hand is more effective.
Word association is very useful way of increasing your Romanian vocabulary - especially if you already speak another romance language as there is an enormous cross-over between them, and this is particularly useful for your passive vocabulary.
Finally, and once at an intermediate level, there is reading, which is one of the best ways to improve, grow and maintain your vocabulary.