danielcg:... and the word-joining (I've just made an exception, translating "vortokunmetado" into English, since I don't know of a similiar concept in this language...

In English, it is commonly referred to as "word-building".

In regards to the original question, I guess the main factor you have to think about is how convenient it is for you to 'think' in such a language. For example, your mind is unlikely to think with 'esperanto' if you aren't readily exposed to it, or if there are English words better suited to your thought processing. Same goes for other languages. I occaisionally, even though being at beginner level and not practicing at all, use some Russian slang and words instead of English words simply because I feel that they suit me better. E.g., I'll go 'Kak intyeryesna' instead of 'How interesting...'. In the same there's probably a 50% chance I'll think of a 'pilko' (even though I'm a beginner at Esperanto) because it is easier to imagine the pronounciation in my head than the English 'ball' (Which can be pronounced around 5 different ways in English ). At least in Esperanto, I'll know there's only one 'proper' way of saying it, and because of that I'm more likely to use that in my head because I avoid being distracted by random stuff like 'how do you pronounce ball?'.

And, for those who want 5 different ways to pronounce ball:
Bull, Buw (with Dark L), Boll, Bow (with Dark L), Bohll

ceigered: I occaisionally, even though being at beginner level and not practicing at all, use some Russian slang and words instead of English words simply because I feel that they suit me better.

My wife is a fourth generation Croatian-American. Her greatgrandmother came over from the old country. She doesn't speak the language, but to this day, her entire family still says "cekaj" (wait) when they want to get your attention. When we first met, I had to look the word up to know what she was talking about. I find it rather endearing, but it is also amazing how long such things will continue.

Being Pennsylvania Dutch, my family still "outens the lights", and hits "shushly" children who "rooch" around at the the dinner table. I say such things without a second thought, don't speak a word of German, and constantly draw strange looks from my wife. I didn't even know it was a strange dialect until I went to college.

andogigi:I didn't even know it was a strange dialect until I went to college.

In Australia we have the same thing, except our old linguistic traditions are much more widespread because Anglo-Celtic Australians make up the sheer majority of Australians. So we say 'ta' instead of 'thanks' quite commonly. But I'll return now to the topic. If you were to WANT to think in Esperanto (it's up to you), the best solution, as one chap said, is to immerse yourself in the language, or use slang from the language. However, you sometimes run the risk of alienating yourself this way if your mates hear you and go 'What the???'.

I would definitely not go around using Esperanto "slang" around non-Esperanto speakers. It will only make you "think" in a very few words, and it will probably annoy your friends. A friend of mine who lived abroad for several years would drop in many words from other languages into her English, and it drove me crazy. I couldn't decide if she was doing it to sound "sophisticated" or because she genuinely couldn't help it, but I was pretty sure that if she tried harder, that she could help it. Maybe this is how she speaks with her English friends abroad (just as we Esperanto speakers might drop in some Esperanto words when speaking another language with Esperanto-speaking friends) but you have to tailor your speech to your audience.

In any case, thinking in Esperanto fortunately deals with your thoughts, not with your speaking. It doesn't have to have anything to do with speaking aloud. I think it is helpful if you translate your "internal monologue" into Esperanto. For example, let's say you look at a thing and you don't know what it is. Try translating "What is that?" into Esperanto in your head. Then next time you see something that you don't recognize, try again, think to yourself "Kio estas tio?", etc. The more common thoughts and ideas you have will become automatic to you. But more importantly, you will get good at having the Esperanto version of an expression come to you quickly - which is useful in conversation.

erinja:In any case, thinking in Esperanto fortunately deals with your thoughts, not with your speaking. It doesn't have to have anything to do with speaking aloud. I think it is helpful if you translate your "internal monologue" into Esperanto. For example, let's say you look at a thing and you don't know what it is. Try translating "What is that?" into Esperanto in your head. Then next time you see something that you don't recognize, try again, think to yourself "Kio estas tio?", etc. The more common thoughts and ideas you have will become automatic to you. But more importantly, you will get good at having the Esperanto version of an expression come to you quickly - which is useful in conversation.

Thanks erinja, this is the kind of thing I was thinking of when I posted the question.