The best tip i'd give is to make your mouth a full 'O' shape when pronouncing heavy letters of tajweed, and to make a Smile face (with mouth slightly open) when pronouncing the lighter letters. Try it. i.e. Say 'Ra' with the mouth fully Open in the 'O' shape, whatever sound comes out will be correct inshaa' Allah. The best basic practise is with letter Saad (full 'O' mouth shape pronunciation) and letter Siin (with a smile shaped mouth which is slightly open.) You should notice this difference and tajweed becomes alot more easier inshaa' Allah.