I send a lot of parcels and whether it be ebay items that I've sold or birthday gifts for friends that don't live locally I've found through experience that there are certain rules you should follow to ensure your parcel gets to it's desired location safe and on time.
My first tip is don't always rely on Royal Mail, there are other parcel delivery services like My Parcel Delivery which are just as reliable and sometimes cheaper. All you have to do is pop onto their website and enter the details of the parcel you would like to send and they give you a quote, if you're happy with the price then with a few clicks of the mouse your parcel will be ready to go.
My next tip is to make sure your parcel is packaged properly, if your item is fragile make sure you put adequate padding around it but choose your padding wisely as depending on what you use you could make your parcel quite heavy which in turn means you will pay more to send it and make sure you label your parcel as fragile.
Add a return address to your parcel, this will ensure that if there is a problem with the delivery and for some reason the parcel didn't make it to the address you intended it to the parcel will then be returned back to you.
If you want to bring the cost of postage down then always make sure whatever you're posting is in a box that is not much bigger than the item you're sending, the bigger the box the more you'll pay, also consider sending things second class instead of first as this will save you some money and you parcel will only take an extra day or so to get there.