This is a good question and deceptively simple to ask, but full of possible responses.

I think that for the beginner it is important to construct simple sentences, with at most two phrases, especially for speaking.

However as one's knowledge improves it becomes more important to construct more complex sentences in order to explore new grammatical structures and new vocabulary.

I study italian and I have found that certain newspapers/magazines e.g. La sette (part of Correiere di sera) have articles with hideously long sentences, where the main subject, verb and object can often be separated by many phrases.

This is part of the joy (sometimes !!) of learning a new language - to try to deconstruct this complexity.

But - it is a good principle to at least start with the acronym KISS in mind (Keep It Simple Stupid !!!)

Yes, this is the famous question ... How can I simplify my thinking? Or: SHOULD I really simplify my thoughts? I am sure learning of languages is a long-lasting process and needs a constant acquisition of growing complex experiences and abilities. It almost always happens to me that I try to express something and come to a point where I have to ponder either about the fitting word (that just right now is not available or confused with another one) or the correct grammar. Not to mention the pitfalls of pronunciation!!! Either I have to settle for less (in this case the thoughts and speech of a toddler) or I have to live with that uncertainty for a long time.  But I cannot really simplify my thoughts without amputation of my head. :-)