Here is my own two cents worth:
1-State the theorems you cite from the literature clearly using Latex's theorem environment and try your best not to cite it inline in the middle of your proof. Also always give the full address to the theorem (e.g. [5,Thm 3.2.1]). This makes it easier to see if what you are looking for really follows from the citation.
2-Dont count on any "well-known argument", "folklore result", "h-principle", etc. Give a precise statement and also citation if the latter exists. Otherwise state it clearly as an "axiom" (as in the Givental-Kim paper on equivariant Gromov-Witten invariants which was written before the theory had a firm foundation.)
3-If your proof involves a subproof that can be important on its own or if it is something you have any kind of "misgivings" about, state it as a separate lemma so that the whole argument would be easier to analyze.