If this were any other word I'd say just restructure your sentences eg. 'The author states...' but you're better off avoiding this word altogether. It's kinda pedestrian, and sounds more like summary than analysis. It's like always using 'said' in a piece of writing; it makes everything sound flat. The author is always doing more than 'stating.' Maybe they're 'asserting...' (more forceful,) 'haranguing...' (angrier,) 'contending...' (like 'arguing')
Working on your verb/adverb vocabulary should prevent this problem, but you should look into some alternate sentence structures anyway:
- The author's assertion that...
- This is further evidenced by the assertion...
- Thus, the author asserts, ...
- As a consequence of asserting... , the author positions readers as...
- The author asserts in a critical and definitive tone that...
Best of luck!