There are two ways to do this good and which one you choose depends on who you are: an ISP or not.
If you are an ISP you should have a good solid base of customers and be on good speaking terms with your upstream ISP's. If not, you should choose the other option.
non-ISP: you want to multi-home your server and/or network.The best way to do this is by using DNS load-balancing. Simply give your servers two Ethernet interfaces each with it's own IP address, making sure that both are public IP addresses as assigned by your upstream ISP's and that one address is from the first ISP and the other from the second ISP.
Now, if you would connect these ports straight to the two ISP's, the server can be reached on both addresses. In DNS, give your server two type-A records and the requests to the server will start coming in from both ISP's.
But you want to connect more to the ISP's. Simply connect two routers, one for each ISP. Connect the routers together with a direct Ethernet and each into a separate switch with another Ethernet. On each router, insert a static route so that you can reach every IP on each leg of your network without forwarding to one of the ISP's. Now, you can connect your multi-homed server(s) to each switch and balance other stations or network parts over the two switches.
This will work fine. You can use one router and try to make two out of it with tagging etc. but with the price of the MT routers it is silly to do so. But anything multi-homed needs two IP addresses, one for each ISP.
Now in case you are an ISP with plans to conquer the world: you must become autonomous to do that right and all the established ISP's want to see you fail in that endeavor so be sure you have a good relation with your upstream ISP's. If you succeed it will be because you are their customer and you have been nice to them. What you will need:
- AN number: This is your Autonomous System number. It lets you define your routing policy in the global Internet routing tables.
- IP address space: you now have address space from your upstream ISP's. What you want is your own address space.
- BGP routing: routing in your network is always needed but until now it has been a technical thing. BGP routing is a political thing and a technical thing. It is not so difficult (easier than OSPF) but you must get it right or else your network will be cut off from the Internet.
That's it but there will be much more to learn and to deal with. Start with Google & Wikipedia and search for: AS number, LIR (if you're in EU this is RIPE), BGP, Internet Exchange etc. and follow all the links with references.
When this is done, you can assign 1 IP address from your own assignment block to any device and announce it to all your upstream ISP's plus make peering agreements with competitors around you (so you don't need your ISP for traffic between you and the other ISP's around you). Also, when you don't like an upstream ISP anymore, you can go away without returning IP addresses and renumbering because they are your own addresses. This is why they don't like it much when you become autonomous
good luck (start with the non-ISP option because it takes a long time for the other option!)