I know this is old school but it works for me.

I use a pocket sized notebook, write the Esperanto on one page & the English opposite on the facing page. I then use a piece of card to mask the 'answer' and keep testing myself. The final test is to write the Esperanto from the English list to check that I can spell what I know mentally.

The added bonus is I have my own concise Esperanto grammar running from the back of the book, so I can learn grammar as well while I am out & about. The doctor's waiting room suddenly becomes a useful opportunity ....

I recently bought the EAB Mini-Dictionary. Having this (or the Fulcher and Long dictionary, which was out of stock when I placed my order) in your pocket all the time and consulting it every time you see or think of a word that you don’t know the Esperanto for can improve your vocabulary.

Here is a flashcard site for esperanto and other subjects as well.
Quizlet - Esperanto

A few tips:

Don't use words in your flashcards, use complete sentences. (Eo on one side, translation on the other). When you can, use Esperanto definitions of "unknown" words on the flip side rather than an english translations

Get sentences from the things you read. There's a lot of Eo content online. When you find sentences you like, with unknown words, copy and paste them into your flashcard program. That's the real secret, read...a lot.

Use the picture dictionaries here on lernu. It's like being able to review several cards at once. They're a vastly under-appreciated resource in my opinion.

Another thing you can do, is practice word building. Take a root word and a a list of affixes. Try to make as many different words as you can. What affixes make sense with the word. What would it mean to turn the root into a verb? a noun? an adverb? etc... What other root words could you combine with it?

Another thing to do, listen to Esperanto audio and transcribe it. Radio Verda would be a good place to start. Just listen over and over until you can figure out all the words and look up the words you don't know. Then listen again. Eventually you'll be able to listen to the entire podcast and you'll know all the words. Then go on to the next podcast and do the same thing. At first it will take you forever, and you may only be able to figure out 70-80% of the words. that's o.k. you'll get better in a few weeks, a lot better. This will also greatly help with your listening comprehension.

1Guy1:I know this is old school but it works for me. I use a pocket sized notebook, write the Esperanto on one page & the English opposite on the facing page..

I do something similar, except that my notebook is electronic - a document in two columns, so that I can open a new blank window and resize and drag it over the column I wish to cover up.

I've done that 2 column thing, too. I usually go through and put another tab in between a pair of words I don't know, then I only study the most tabbed words on the next pass.

Here's something that I wrote a few years ago: Ekzerco. Nothing fancy, no history, doesn't show the harder words more often or keep track of your progress (except a percentage for the current session). But I like the multiple choiceness of it. There are six sets of words to work with, set 1 containing the most commonly used.