Getting used to the sounds in any language takes practice. Getting used to them in a tonal language when you are coming from a non-tonal language is a bit tougher, but as with anything else, it is easily accomplished with a bit of time and effort.
One simple piece of advice that can make a big difference in your pronunciation is to emphasize the last tone of a sentence and to a lesser degree of a stop in a phrase. Open your mouth a bit when you talk. Pay attention to what native speakers are doing with their mouths now and then.
Know how to say 3 ? Know how to say it correctly? Your mouth should be opening wide at the sides in a big smile-like expression.
Look at the following sentence – พรุ่งนี้ ไม่ว่าง (tomorrow – not free)
There are 2 great opportunities to make your speech sound ชัด here. Hold that high toned นี้ in พร่งนี้ and make it long and hold it for a moment. Not too long, but long enough that its a clear high tone. Then on the ว่าง of ไม่ว่าง…remember its a long vowel and its falling. The sounds we hear right before a pause are more memorable than the ones that precede them and when you are still trying to climb up that slippery language ladder, little tricks like these will make you sound ชัด beyond your ability bringing the praise that motivates you to keep going.