keigo - Use of -です／-ます in the middle of a complex sentence

I have always thought that -です／-ます should be use only at the end of a sentence but some japanese friends told me it sound more polite to use it with every verbs.　What should I say between, by exemple:

映画を見に行くので、今日は会えません。　(this one seems more natural for me)
映画を見に行きますので、今日は会えません。

道にいる男は面白いですね
道にいます男は面白いですね　(this one feels definitly wrong)

彼は弟だがそんなに仲が良くないです。
彼は弟ですがそんなに仲が良くないです。 (and this one sounds to be the correct one with me)

do you know the precise rule?

Answer

Basic rules are:

• 
  

  One polite marker per main clause
  Beware that it's not "per sentence", since a sentence with coordinate conjunctions is deemed to have multiple top-level clauses tied with them.

  
  

  
  

  〔彼は弟だ〕が〔そんなに仲が良くない〕 + [politeness]
  → 彼は弟ですがそんなに仲が良くないです。 (polite markers retained)
  × 彼は弟だがそんなに仲が良くないです。

  
  

  〔〔道にいる〕男は面白いね〕 + [politeness]
  
  → 道にいる男は面白いですね。 (no polite marker for subordinate clause)
  × 道にいます男は面白いですね。

  
  

  
  

  However, there's a bit old-fashioned honorific usage you'll sometimes hear, which puts polite markers even to relative clauses. Basically you don't need to follow that custom unless you're working in high-rank service industry or speaking like a Victorian lady.

  
  

  Additional examples in: Masu te-form with Kudasai?

  
  


• 
  

  Polite or non-polite should agree in a sentence
  If a sentence has multiple main clauses, the politeness should agree. You can't mix plain and polite forms together.

  
  

  
  
  

  ○ 彼は弟ですがそんなに仲が良くないです。
  × 彼は弟ですがそんなに仲が良くない。
  × 彼は弟だがそんなに仲が良くないです。
  ○ 彼は弟だがそんなに仲が良くない。

  
  

  
  

Note that some frequently used conjunctions, namely ので, のに, たら and と, have ambiguous identity between coordinator and adverbial clause marker, thus accept both polite and non-polite endings in polite environment.

---

Further readings: 従属節の従属度と丁寧形, 従属節における丁寧体コピュラの自然さ