Learn to play drums - seriously - it'll do wonders for your feel of rhythm. Even just learning a simple 'beat' and being able to maintain that beat for 3 or 4 minutes at a steady tempo will help calibrate your 'inner metronome'. Also - practice 1/4 notes, 1/8th notes and 16th notes in your head when you're out walking anywhere or even do step exercises. As humans, we tend to walk in time - it's pretty impossible not to.
Also, what I find works when sight reading is seeing 'patterns' of phrases and rhythm's. Music in general can be pretty repetitive and you'll find that certain rhythmic phrases continually crop up. Learn to recognise these patterns when reading ahead and all you really need to concentrate on is the notation.