1. Say the word out loud. You've heard the new word from your teacher, or read it somewhere and you understood its meaning... you may write it down and you are done. This step is makes sense and is very simple, but it is sometimes forgotten. Take a couple of seconds to repeat the word. Make sure you are comfortable pronouncing it. Then cover the word (if it is written) for a couple of seconds and see if you can remember it without looking (repeat until you can say it, at least just a couple of seconds after covering it up). See if you can say the word out lout after a few minutes have past.
2. Put the word into context (1): make a sentence. The very next thing would be to make up a sentence (or several). To focus on your new word, good sentences would be mostly formed of other words you already know (plus the new word). You can get started with any old sentence, but as you go own, try to make the sentences as realistic and as relevant to you as possible. Say you learnt the word avô (grandfather), then make a sentence about your own real grandfather (e.g.: O meu avô mora em Nova Iorque. or O nome do meu avô é John Smith.).
3. Put the word into context (2): visualize. You can try visualize the word by itself as you say it, but also think of the sentences you made in the previous point (you are more likely to use the word in a sentence than by itself later). You may visualize the meaning of the sentence, but, most importantly, imagine a context when you would actually say the sentence. For example, you may imagine the following for the word avô:
A: - Mary, esse é o meu avô, John Smith.
[Imagine the whole situation, introducing your grandpa to a real friend]
A: - Eu moro na California, mas eu sou de Nova Iorque.
B: - Que legal! O meu avô mora em Nova Iorque.
A: - Que lindo bebê! Qual é o nome dele?
B: - John Smith.
A: - Ah! O nome do meu avô é John Smith!
4. Put the word into context (3): use stickers. This is an old trick, but it is good. Say you want to remember the word for refrigerator (geladeira in Portuguese). Stick the word geladeira to your fridge, then every time you get close enough to your geladeira you will be reminded of the word. ... Sure, your avô would probably not like it if you put a sticker on him, but you may get away with placing one on a picture of him.
5. Use it! The more you use a word the more you will remember it, so make sue to get as much practice speaking Portuguese as possible! The more you speak, the more likely it is that you will end up having a chance to use your word.
[Note: You may not do this for every new word you hear... just a few that you really want to or may be having trouble remembering].