It would depend entirely on what your needs are and what type of service you're looking for.
True load balancing can only really be done with two or more connections to the same ISP.
If you are only using the connection to get Internet access and not running any servers, a "backup" connection can be relatively easy to set up, with one line from each provider. It could be done with a simple dialup line through a firewall, or even connected through a router that supports both the backup dialup and your primary connection. Or you could use a DSL line or ISDN line for backup of a T1 line. You would need to be using Network Address Translation (NAT). When the primary service goes down, your router or firewall would initiate the backup connection, and begin using whatever IP address is assigned to that connection. The NAT configuration would allow your computers to continue to work through the backup connection, without needing to reconfigure every machine to use a different IP address.
If however you are running servers, then in order to use a "backup" it's a little harder. Unless you want to always be updating DNS records when your service goes down, your servers always need to be using the same IP addresses. In general, if you have one ISP, you'll have been assigned a block of IP's to use on your network. Those IP's cannot be (contractually) routed through any other ISP, so they can't be used as part of a "backup" connection; technically they can be, but it's extremely unlikely that either ISP would be willing to set that up. In this case, you would need to apply to ARIN to get a block of IP addresses assigned to your company. They only assign a Class C block as the smallest group, which is 256 addresses (254 usable). Once you get the IP block, you would work with your two providers to set up your connections so that normally those IPs are routed through your primary service, but if that goes down, your secondary connection is activated and immediately routed through the secondary ISP, and the primary ISP must stop routing them until the service comes back up.
If you wish to have a "multi-homed" connection, you will also need your own block of IPs from ARIN. In this case, you would have two connections active, one from each ISP, at all times. Both would actively route the IP block you have; other routers and servers on the Internet would receive both routes as possible paths to reach your network, and the software in the routers will choose the best path based on criteria their owners decide (such as whether one path goes through a link that they have to pay per-megabit, or through a path that they have unlimited bandwidth usage on). Your router will receive incoming traffic over both connections, and when your computers send traffic out, your router will make the same choices about which path to use. If one connection were to go down, that ISP would stop advertising itself as an active route for your IPs, while the other would continue working as it had. For a short time, some routers on the Internet would continue to see the route to the "down" service as usable, which will result in data loss, but updates propagate pretty quickly, so the lost data should be short-lived.
A very good book that will help you make decisions on what types of services you need is called "Getting Connected" from O'Reilly. It explains all of the available Internet services, how they're set up, and helps you through the process of ordering it.
You'll also need to contact your ISP to determine what possibilities you have, and you'll need to contact other ISPs to get pricing and availability of backup or multi-homing services. Many ISPs won't provide BGP services to "small business" connections or allow you to use your own IP block; BGP is the protocol used to allow your to use your own IP block from ARIN through two different ISPs -- it tells the routers at both ISPs that that particular IP block belongs to your router. Many also simply don't have a system setup that allows "backup" service, which would only be charged based on how often you use it, as most connections are intended to be "always on", such as DSL and T1 service; ISDN service is a very common backup solution, as you're only charged a basic fee for the line, and then charged based on how much you use it.
It would be a very good idea to hire a network consultant to help you set up this stuff, if you're unsure what you're doing.
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