practice to improve complexity of speak

I'm in the same boat as you.  But i'm afraid the answer is that there's no shortcut.  Something like writing down the complex sentences, but creating new complex sentences by substituting words in the spaces. 

Even in English, it often occurs to me later that I could have said something much better than I did.  Sometimes that memory makes me better prepared for a similar situation.  So, I think one just needs to muttle through; say what one needs to at the time, and build up a reseviour of experience to draw from.

Hi adamplax, not sure if this helps: When I first became conscious of my bilingualism (mine was Cantonese and English), I started to notice that my thought patterns were different depending which language I was using, and if I am caught in the wrong "mode" eg. thinking in English and speaking Cantonese, speech comes out weird. So the first thing I made a conscious effort to do was to ensure before my mouth opens I was not in the "wrong" mode, and to switch if I was. "Switching" for me was to imagine a TV scene in the right language, or recall a past conversation in that language etc. These days I do it pretty automatically between English and Cantonese.

Over the past few years I have been trying to bring my spoken Mandarin to the level of my Cantonese. I have still some work to do, so I still need to do the conscious mode switching a lot. But I find I get rid of a lot of muddled sentences if I do.

Another thing I do is to practice actually saying it, instead of pretending to say it in my head. I find this important for "muscle memory".I find sometimes my mind knows what to say, even having the sentence ready, but the mouth muscles doesn't execute it properly. So I spend time actually saying the same thing a few times and in a few different ways, recording and replaying it to avoid self-delusion. I used the "Expansion" tab in Chinesepod's lessons a lot for this, repeating Jiaojie's sentence and then saying it in 3 or 4 different ways very quickly. Most times I mess up the sentences when it first appears in my mind, but as I go I get better and better at quickly converting new thoughts in my head to actual sounds from my mouth. Still a long way to go for me, but I find I'm improving quite markedly doing this.

I borrowed this technique from suicide prevention training where we find people mess up asking the question "Are you going to kill yourself?" and the only way to overcome that is to practice saying it out loud again and again. Ok, pedagogically this may not make sense, but works for me!

Hope it is of some help...

I agree with hiewhongliang that it's best to practice out loud. I remember when I was studying at BLCU that my roommate would always practice everything out loud. At the time I didn't really understand this until I started studying myself. I find that muscle memory is important when practicing speaking in a new language because what you think and what you actually say are often times different. If I haven't spoken out loud in a while I find that my tongue gets tied!

There really are no short cuts when learning to speak a language. It takes years to become proficient. I think that the best thing to do is go live in the country where the language you are learning is spoken (if you don't already). That way you are surrounded by native speakers where you are forced to practice daily. With time you find that certain things don't sound right - it becomes just a feeling rather than having to think through all of the grammar rules. 

I wish I could go back to China to study again, but until then I will just have to do my best with what I have here which is mainly just online resouces and my online classes - and my one Chinese friend at my office!