A formal 'yours sincerely' is 'veuillez agreer, Monsieur, Madame, l'expression de mes sentiments distingues' - with 'e' acute accents on the first e of 'ageer' and on the last 'e' of 'distingues''.
Lots of ways: "con cariño" or "con amor" is "with love", or you can sign "atentamente" when it's a formal letter, depends on what type of letter you are writing